It's revealing that so many different Monastery Mentor strategies have been successful. It's evidence of the card's power, and also suggests that some successful lists might be hiding design mistakes behind the raw power of the cards they run.
There's probably four different key secondary cards that it has been paired with. Every list shares commonalities: 3 or 4 Gushes, and 2-5 Delve spells, 2-4 Planeswalkers.
- Remora
- Snapcaster
- Pyromancer
- Delver
Remora and Snapcaster strategies are really control decks, while the Pyromancer and Delver variants are aggro-control. It is difficult for a deck with fewer than 6 creatures to pursue the aggro-control role. Even if one finds a threat on schedule, opponents have enough answers to deal with it without compromising themselves. 6 creatures is probably the minimum you can run to have success pursuing the aggro role. 7-8 creatures is what Delver lists have been running since Khans of Tarkir encouraged replacing Snapcaster and Trygon with delve spells and Dack Fayden. I would be interested to see if anyone tries an aggro-control package using Snapcaster as the 4-of secondary creature alongside Monastery Mentor.
Ultimately, my preference is for whatever approach maximizes Gush, which for me remains the most important card. I have a immediate distaste for strategies that can't run 4 because their other choices (Mana Drain, Jace) aren't synergistic with the card. Personally, if I wanted to play control, I would either play URB Control with Dack, Notion Thief, Tinker, or something UWx that ends with Ojutai or Dromoka swinging away. If Mentor is actually going to outclass Pyromancer, it needs to perform alongside Gush as well as Pyromancer did. To me, that means a combination of 3 or 4 Delver of Secrets and 3 or 4 Monastery Mentor.
Losing role flexibility is not worth the power upgrade Mentor provides. I would rather optimize Gush and my ability to play aggro-control than optimize Mentor. That might be a real choice one has to make. It's not clear one can optimize both Gush and Mentor while maintaining role flexibility. The question then is whether running a suboptimal Mentor is better than Pyromancer in an otherwise near identical shell.
Incidentally, Oath's evolution to Show and Tell and the presence of Mentor in the format means that white is likely now preferable to green for Containment Priest and Swords to Plowshares over Cage and purely running Bolt. That does not mean one goes straight to Mentor, but staying URg is possibly worse than URw, independent of the Mentor/Pyromancer choice. As the metagame adjusted to Delver's dominance with more oath and more big creatures, even before Fate Reforged, we were already seeing a slight move to URw. The upshot is that with less Ancient Grudge and Nature's Claim in the format (replaced by Wear/Tear and Shattering Spree), Workshops becomes slightly better. Tangle Wire also becomes marginally better, as Gush lists have fewer instant speed ways of dealing with artifacts. I think the shift in Workshops from Terra Nova's reliance on spheres to Martello's Tangle Wire and Combo element is mostly explained by Gush aggro-control becoming predominantly URw and Pyromancer becoming Mentor, and I think top 8s have more or less born this out over the past few months.
