As evidence comes in, opinions and conclusions must change. To hold onto them in the face of contrary data is childish stubbornness. However, said opinions shouldn't just be thrown away, either. Every piece of evidence must be critically evaluated. If the data is questionable, then any conclusions based on it must also be considered questionable. Valid data and results should be considered contrary evidence and used to reevaluate previously held ideas.
Case in point: A bit over a month ago when I covered Liliana of the Veil making it into Pioneer, I predicted that she would lead to Mono-Black Devotion becoming more of a thing than it currently was. So far, I've been kind of right. That also means I've been kind of wrong. Why this has happened says a lot about Pioneer as a format.
Dominaria United brought a surprising number of mono-black cards, and players were quick to take advantage. Liliana was obvious, but Evolved Sleeper was a nice pickup as well. A solid early beater, it's also a great late-game mana sink for Nykthos, Shrine to Nyx. What I wasn't expecting was Sheoldred, the Apocalypse making waves. It turns out that a 4/5 for four with a punisher mechanic is quite solid. Individually, these cards were quite good, but together with Gifted Aetherborn built a very solid devotion curve, and players tried to take advantage:
When I said that Mono-Black Devotion should exist in Pioneer, this is what I was thinking of, though there are some odd choices. I have to believe that Warlock Class is only there as a Nykthos sink, the effect is pretty small otherwise. Similarly, maindeck Leyline of the Void can only be explained as free devotion, as the metagame barely calls for graveyard hate. There are a bunch of other numbers I'd quibble with too, but overall this seems like a solid way to build the deck.
The problem is that this doesn't appear to be working out. Dedicated black devotion decks have had extremely limited success. Mono-black decks are overall seeing more play than pre-DMU, but they're hardly setting the world on fire. If Nykthos sees play at all, it's as a one-of.
It might be easy to brush off the lack of Nykthos in this deck as a function of too many colorless lands given the Field of Ruin. That is quite similar to why Standard Mono-Blue Devotion only ran one, after all. However, again, this deck is an aberration for playing any Nykthos, so clearly there's a problem with the card. One that cannot be solved with Urborg, Tomb of Yawgmoth.
The obvious reason for this faltering launch is that there's already a strong midrange black deck in Pioneer. Yes, it's actually a Rakdos deck, but it's not an equal RB split. It's more rB, with far more mono-black cards than mono-red ones.
Rakdos Midrange has been a solidly performing deck in Pioneer for quite a while now, and the addition of Liliana has largely done what I expected. Gone are the other cheap planeswalkers and the deck is far more attrition focused than before. With this pedigree, there would naturally be resistance to a new deck in the same metagame niche.
The Main Advantage
However, a strong enough game plan would win over converts regardless of entrenchment. The fact that this didn't happen is instructive, and it comes down to the irreplaceable red spells. Surprisingly, these are neither Kroxa, Titan of Death's Hunger nor Bonecrusher Giant // Stomp, whose primary functions can be replicated by another Liliana and Murderous Rider // Swift End respectively. Not perfectly, to be fair, but close enough to make only a minor difference.
No, the problem is that there's no good replacement for Bloodtithe Harvester or Fable of the Mirror-Breaker // Reflection of Kiki-Jiki. I found this very surprising, but my testing and testimonials from other players confirmed this conclusion. It's an issue of velocity. While the bodies that both cards are quite relevant, the main draw are the rummage effects. Harvester's blood token and the second chapter of Fable are the glue holding the whole deck together, and while there are alternatives available, none are as efficiently powerful as Harvester and Fable.
As everyone I've ever discussed Pioneer with has said, there's something inherently clunky about Pioneer. It represents a swath of often contradictory design philosophies in Magic's history and lacks a solid anchoring identity. Legacy has blue cantrips and Force of Will. Modern has its fetch/shock lands manabase. What is Pioneer built around? I don't know, and nobody I've asked agrees on anything.
A lot of this is that the card pool is quite weird. There's a lot of power concentrated in a few sets with wildly different design philosophies. Khans of Tarkir was card advantage and card draw rich with powerful three mana or more threats while simultaneously being poor on answers. Streets of New Capenna has great low-end threats, looting effects, and answers. The power's all over the place and the deckbuilding options are wildly inconsistent.
Consequently, any deck that wants to can draw lots of cards, but generally as high-end effects like Treasure Cruise. Cheap ways to smooth out draws are quite thin. Any deck that has access to looting or rummage effects has an advantage over those that don't, and this is true for Rakdos vs Mono-Black. The latter has strong card advantage but getting to the point where it can actually deploy it is harder than Rakdos' cheap rummaging. Thus, Rakdos is better at overcoming Pioneer's innate clunkiness than Mono-Black and is rewarded with more and better results.
The Nykthos Problem
All of that goes a long way to explain why Mono-Black Devotion isn't catching on, but it doesn't help with why Nykthos sees relatively little play. Powerful, colorless mana engines are usually more widely adopted but that isn't true of Nykthos. My testing showed that it can work in non-green decks, but nobody really tries. So, I asked several players working on mono-black decks at my LGS whether they'd tried Nykthos out and what they thought. While each had multiple answers, there were two in common that were illuminating.
#1 Nykthos Is Clunky
The main point that everyone agreed on was that too many Nykthos made their deck clunky. There were too many hands rendered unkeepable when the only lands were Nykthos. This wasn't helped by Urborg because drawing too many of those is equally bad. Without Expedition Map there's no good way of ensuring that the critical cards are found without having to play too many and therefore make the deck inconsistent.
#2 Nykthos Is Kinda Useless
This was a shock to me, but they all agreed that they didn't actually need Nykthos, so it wasn't worth risking the above clunk. While a pile of mana is always great, their testing showed that they didn't need to deploy more than two spells a turn, so paying for them via normal lands was always an option. Using Nykthos as an investment for their top end sounded like a good idea, but they...don't really have one. Invoke Despair is the best curve topper any of them had found, and at five mana isn't something for a Nykthos deck. Consequently, Nykthos didn't do anything, so it got cut for other utility lands.
At Last, An Answer
That last point has finally answered my longstanding question. There aren't more Nykthos decks in Pioneer because only green can make use of the mana. There's no reason for a blue devotion deck to exist, Spirits is just better. Black and red have devotion decks but they can't really do anything with Nykthos as they don't need to build mana for a massive endgame. They're perfectly fine chipping away at their opponents with already affordable cards.
Green devotion, on the other hand, needs Nykthos to power its game plan. While initially, Mono-Green was a straight beatdown deck, it's now more of a combo deck that beats down as a backup plan. It needs the mana to power out Storm the Festival and then power its Chain Veil combo finish. The ramp is meaningful and the mana useful.
To be fair, the answer was perhaps sitting in front of me this whole time. When Walking Ballista was legal, green devotion and white devotion were both good and players were more willing to make red and black devotion work. Ballista was the game-winning mana sink Nykthos needed, and anyone could use it. Once Ballista was banned, devotion collapsed down to green because no other color needed that much mana. Nykthos is a combo card, not a value card.
It Is What It Is
Pioneer is very strange. It represents an extremely volatile time in Magic's design history. Consequently, there's some tension and inconsistency in the card pool that players just have to deal with. I thought that said card pool should push towards more Nykthos-powered devotion decks, but I never appreciated how most decks just can't make use of the card. I remembered Nykthos as a value engine, but in Pioneer's context, it's more of a combo card. I'll just have to get used to this new reality.