YMMV / Monster Train

• Game-Breaker: As the game is still being actively-balanced, there are certain builds that absolutely destroy the game's difficulty:
  • Petty Theft, a spell you can get from a Frosted Caves event. It does a ridiculously small amount of damage... but nets you 50 gold if you kill something with it. Add extra damage and Holdover (easier with an Artifact that gives spells an extra upgrade slot) and you might as well rename it "Infinite Money."
  • Melting Remnant's Little Fade champion is infinitely replayable and has an upgrade tree that's all about it getting preposterous bonuses whenever it dies. Granted, Fade is extremely fragile on this upgrade path, but that only helps it die and get its stat boosts, and the upgrades also give it Quick, ensuring it can get an attack off before dying, and after a death or two it'll be strong enought to one-shot anything but the boss. Its attack can easily be in the hundreds by the time the boss appears, and while said boss will almost certainly kill it in one hit, put a durable unit or two in front and it'll shred the boss before the boss can even get to it.
  • Any combination of the Split Anvil, a Spell-heavy deck, and a plethora of Incant units (usually via the Stygian Guard.) Since Split Anvil makes every spell that costs less than the last you played free, opening with a strong spell like Titanstooth will likely give you a variety of Spells you can now spam for no cost. Setting aside the advantages of dropping multiple expensive buffs or damage spells all at once, any Incant units on that floor will also trigger their effects every time, leading to spikes in Armor, damage, health, and even things like slapping the enemies with tons of Frostbite. Pair it up with Spells, Units, and Artifacts that improve Incant, hand size, or allow for more drawing, and it's possible to obliterate Seraph on the first floor before he even kills a unit.
  • Umbra is by far one of the strongest factions in the current incarnation of the game. Their units are merely good, but morsels can allow them to scale considerably as the battle wears on. On its own this is potent, but not broken. The issue comes with the infamous Morselmaster + Morselmaker combo, two low-cost, low-capacity units that are unspectacular on their own, but will double morsel spawns and space two free morsels every turn, respectively. Pair them up with an Umbra unit that has a strong Gorge ability in front and watch the snowballing begin, without the player having to lift a finger; they can then focus their Ember on other matters as that one floor spirals out of control without any need for intervention, until it's capable of soloing even the boss. For the ultimate in broken fun? Make the unit in front the Overgorger - get this combo rolling early, and by the time you get to Seraph, the Overgorger will be powerful enough to one-shot nearly anything.
  • The Last Divinity DLC gave us Reap. Reap is basically damage over time, but unlike Frostbite, it doesn't fade. Sure, Reap usually comes in smaller amounts than Frostbite... but the Wurmkind clan specializes in it, and it's not unheard to have cards that inflict 20-30 point of Reap each!
    • To elaborate, Reap scales with the Clan-mechanic Echoes, allowing each stacks to deal increasingly more damage based on the echoes on the current floor. It's not unheard of for Reap-centric builds to wipe out flying bosses long before the Relentless phase!
  • Also from The Last Divinity is the Melting Remnant's new card Hallowed Halls, which kills all friendly units on a floor and reforms dead units at random until you're over capacity. With a bunch of small units (ie. Imps, Dregs, Draffs and Tomb units, the latter three of which you're sure to have plenty of) you can potentially do it every turn and have a heavy-hitting, very hard to kill floor that's capable of shredding most enemies.
• Goddamn Bats:
  • Clipped Reflectors. They get 10 armor every time you cast a spell on their floor. Got a spell heavy deck? Either you basically discard your hand without playing it, or you turn the Reflector into unstoppable meat shields. Piercing spells turn easily dispose of them... but you need to have those spells.
• It's Easy, So It Sucks!: But only initially. Where a lot of roguelike games start out at a high level of difficulty and simply getting to the end once is a challenge, Monster Train instead starts out pretty easy and only ramps up the difficulty on higher Covenant levels. This can lead to a player succeeding on their first or second run and the victory feeling unearned and underwhelming, and only if the player carries on will they discover how hard the game really gets.
• It's Hard, So It Sucks!: The Last Divinity DLC was polarising, with some players arguing that the huge buffs pact shards give enemies make a lot of strategies no longer viable due to increased enemy health and damage as well as additional abilities (e.g. Sweep making it impossible to protect fragile units with meatshields).
• That One Achievement: "How to Deckbuilder", which requires you to clear a run with no cards in your deck by the end. In a game with an unusually high global achievement completion where the highest achievement ("Early Survivor", for defeating the first major boss) has over a 98% global completion rate^note more impressive than it sounds, as even games with achievements like "Start the game for the first time" don't tend to reach a 100% global completion because of players who buy the game and then never even play it bringing the global average down, this one has only a 1.3% completion rate.
• That One Attack: Enemy units with Sweep hit every unit on their floor, meaning they can bypass your meatshields and take out fragile units in the backlines. This makes it extremely risky to build your strategy around low-HP units (e.g. Tethys Titansbane) if you don't have some additional way to protect or resurrect them.
• That One Boss:
  • The Crystalcloak, a boss that may appear on Ring 5. They have eight stacks of Stealth, meaning that your units will skip their first 8 attacks. The boss themselves don't do a ton of damage, but unless your build is skewed towards high-hp units and/or you have some seriously hard-hitting spells to take out the boss, you will be forced to watch as the Crystalcloak relentlessly shreds your people and gets to the Pyre with their health pretty much intact... which usually ends the run.
  • Unfortunately one of the variants for Ring 5 is the Self-made Harpy, who can be every bit as bad. Their gimmick is five stacks of Multistrike, which is devastating to any build reliant on lots of small units or glass cannons. Multistrike is in general so powerful that most enemy units that have it are very fragile to compensate... but as a boss they shrug off precision spells you've laid aside to deal with those. They get even worse on later Covenants since they are accompanied by Sycophants, who boost friendlies' attack on death.
  • The Diligent version of Seraph can be cause for an immediate reroll if you see it come up. There are some decks that can cope with all of their spells being consumed over a protracted battle, but they're quite specific and for the most part it's not worth your time watching your deck get steadily destroyed trying to defeat him with anything else.