YMMV / Under Mine

• Demonic Spiders: Mimics. Super aggressive with a very tiny window of vulnerability (essentially only when they land), their only real saving grace is that they are usually well-telegraphed by showing up as chests in rooms with no chest icon, but they can show up in rooms with multiple chests where it basically becomes a coin toss.
  • Imps. Hard to hit and will spawn delayed explosions right under your feet, the attack being disruptive enough that you virtually need to deal with them first, which can be difficult in conjunction with other enemies.
  • Firebats. Take the already annoying bats, give their hard-to-avoid sonic attacks even more range, along with a massive increase in health pool and the ability to spread fire wherever they fly and you have a supremely annoying enemy type in smaller rooms.
• Good Bad Bugs: If you acquire the Mushroom relic (+1 max HP, heal one HP for every enemy killed) before exiting the Dungeon and run across a Crossbowman, you can gain a kill for every bolt shot down. So if you're patient, you can basically acquire as much max HP as you want. This bug was fixed at the full game release.
• Gamebreaker:
  • Blessings in general. Simple, yet Awesome direct improvements to performance, stackable, and relatively-common to acquire. While powerful Relic synergy is always a staple of games like this, many players can find just as much satisfaction in finding and adding yet another stack of Hardiness or Exuberance - with far less rarity and effort than the former takes.
  • The sylph is so much more powerful than the other familiars that there's little point in using any of them barring maybe the djinn. While the others may be an additional source of damage or allow you to use potions longer or whatever, the sylph allows you to use penance at half costs, use altars a second time and doubles how many blessings they apply. Combined with Holy Guacamole and Devotion, you can get 12x the levels of blessings as not having any of these. Even if you have these relics with another relic, the sylph still provides triple the amount of blessings.
  • Using the Ambrosia potion doubles a random blessing and isn't a super rare potion, though it's not common either. Still, getting 2-3 in a single run is normal and more isn't out of the question. If you only get a few kinds of blessings, this means you can make it very likely to double the effect of a really valuable blessing for absurd results. Let's say you have 50 stacks of melee damage blessing, which is not hard at all when you have the sylph. Double that. Double it again. Double it a third time and you suddenly have 400 stacks of attack damage, easily enough to oneshot literally any non boss in the game and to melt the bosses in no more than a few seconds.
  • Certain relics have insane synergy with each other. For an example, look at the Obsidian Blade, Master Pickaxe, Soul Guard and Ursa Major. The first two relics have very powerful effects that are balanced by the player being unable to use them if they take any damage. Soul Guard blocks this by reducing all damage taken by 80% and then having all damage come out of max HP, not total HP, meaning your HP never actually decreases when you get hurt. The final demerit of this combo, your falling max HP, is balanced out by Ursa Major giving a temporary boost to maximum HP. So long as you keep eating, you get double damage, ranged melee damage or both plus huge damage reduction plus incredibly high HP. Improving this basic three to four part combo is Leftovers, Seasoned/Caramel Popcorn, and Butcher's/Four Leafed Cleaver.
• Scrappy Mechanic: Many players find it irritating that using a bomb will reveal if there's an entrance to a secret room nearby, but not whether there's a crawl space hidden beneath a rock. If you want to find one, either you bring the Djinn or just start bombing at random from one of typically six to twelve rocks on the floor. Making it worse is that there are several recipes that can only be found in crawl spaces, including the Rat Bond, Hungry Ghost and All Potion. Fortunately, the crawl space containing Lilyth is at least marked with a very visible X, though you have to know to look for it.
• Scrappy Weapon: In a game where every Relic services to empower the player and make the run easier, there is no one that will inspire as much bile and immediate, on-sight revulsion as the Galoshes. This upgrade grants a slight boost to movement speed (which is fine, but underwhelming for a Relic), immunity to the impairing effects of Oil (a trivial boon, at best) and...causes the player to leave a splatter of oil whenever they land from a jump. Without fire immunity. There are very very few scenarios where this would be beneficial (as ground fires deal pitiful damage to enemies, anyway) and many where they can be fatal to the player (for starters, Seer's fight becomes a nightmare because every jumping dodge will create a new fire hazard at the landing point.) Picking up the Galoshes can effectively neuter a run, forcing the player to either avoid using the super-useful jump, pay extra-careful attention to their movements (in a game where information overload is already a problem in the more hectic encounters), or else accept inflicting tons of self-damage through self-inflicted fire hazards. Some players despise the Galoshes so much that they'll dump it on the Cursed Altar, sacrificing a useful game mechanic just to ensure the Galoshes can't show up anymore. The only way to make it halfway is okay is to pick up the Lavawalkers, a pair of boots that combines with the Galoshes to make you immune to fire and leave flaming patches on the ground. Even then it's just okay.
• That One Attack: Seer has one attack where it will spawn a mass of bouncing black balls that will not disappear until they hit either you or the boss. Guess which will likely happen sooner?