These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: The "back to basics" approach of Might & Magic X: Legacy (adapting the old-style system of play with a modern feel) has a few fans - mainly younger ones - up in arms. Many of them would like to see the free movement system of the later games like VI thought VIII and the turn-based system done away with; these fans are missing the point completely, being Real Life examples of folks who are Wrong Genre Savvy.
Demonic Spiders: Shows up in nearly every game. There are enemies that age your characters (nigh-impossible to reverse), enemies that drain your MP to empty in one hit, and enemies that flat kill you (or worse) in one shot.
Instant death effects should not be a problem once you level your healer, in VII and VIII at least, to cast master Protection from Magic. Just keep an eye on the spell icon in case it ends prematurely. Mana drain effects, however, can very swiftly neutralize your ability to heal your party, turning moderately threatening enemies into this.
The mana drain effect is almost a case of The Computer Is a Cheating Bastard with regards to the Final Battle in 6. Simply put, after you destroy the Reactor in the Hive, you have to fight a lot of Devil Kings, Devil Soldiers, and the Demon Queen, the Final Boss. Here's what makes this setup hard: When you destroy the Reactor, all spells currently on your party are automatically dispelled, leaving you vulnerable, and Devil Kings and the Demon Queen have the mana drain attack, meaning you have to recast them before a dozen attacks from them leave you completely unable to do so. It isn't easy.
Then there are Eyes, which are really bad, especially in 6. Flying Eyes can cause Sleep. Terrible Eyes cause Fear. Maddening Eyes cause Insanity and can cast Dispel Magic, which quickly robs you of your protective spells. What's worse, each has a different immunity, so if you try to use a spell on them that effects a large group of creature (like say, Fireball) you can't hurt all of them. All three are very vulnerable to Poison, but the only good spell in the game that does Poison damage is Dragon Breath. Even worse, Castle Darkmoor (a dungeon that you have to go to for an obligatory quest) has hordes of these things, and for some reason, they can attack you through walls.
In VII, this Trope definitely applies to Robert the Wise or Toberti. (The one you have to fight depends on whether you're on the Dark or Light Path; cosmetic differences aside, it's the same Boss.) This guy not only has a lot of hp and powerful defenses and a few immunities, he can cast Hour of Power and Power Cure on himself, and his attacks can cause Eradication. (The best way to deal with this Boss Fight is Save Scumming, and save each time you score a good hit.)
It's also possible to steal the item you need from him without having to deal with the fight. If you cast invisibility and have a character with Grandmaster Stealing, you can snatch it right off him and he doesn't know you're there. What's funny is that the one who gives you the quest to kill him and get the item still assumes he is dead.
In X, the Limbo dungeon has something called Alien Technology, which looks like the Ubisoft insignia. This Boss has a million hit points - estimated - is highly resistant to magic, immune to conditions, and has overwhelming physical defense. It can't attack you physically, but it has an ability that damages every character by half the damage it receives from physical weapons. (The best way to kill it is probably to stand two squares from it and shoot arrows at it, although this does take a long time.
Both the bosses in Fort Laegaire in The Falcon and the Unicorn DLC qualify. While they are about equal to your own party in level, you are severely handicapped because in this part of the game, your gear has been confiscated and you have to use the second-rate stuff provided by the Blackfangs in the brig.
The first one, Perren, has two Mooks helping him; one of them can inflict the Poison condition and the other can inflict the even worse Feeblemind condition. There's no way to obtain any potions that can cure these conditions here, so you have to depend on luck and your own healing spells. Perren himself can cast Fireball, and the only real defense against this is Fire Ward, if you can cast it yourself.
The other one fights alone, but he's stronger, using Sun Ray and Pacification, which can cause the Sleep condition. Fortunately, once you defeat Perren, the Blackfangs sell some better equipment, and they also sell some Light Ward scrolls, but the only real defense against Pacification is the spell Burning Determination, if you have access to it.
That One Dungeon: A lot of dungeons are like this, sure, but one that really stands out is the Breeding Zone in VII. See, most dungeons have a respawn time, meaning if you kill the monsters, it takes them a while to reappear in it, usually about three months game time for dungeons. The Breeding Zone, true to its name, isn't like that. It respawns completely every time you leave. That means the usual means of handling a dungeon, clearing out a little at a time, then leaving to rest and regain your health and spells, won't work here. And to make it worse, the monsters in this place are tough. You'll have to deal with Gog, Fire Elemental, and Behemoth type monsters. The Behemoths are miles above you level when you first go into the Breeding Zone, capable of one-shotting your character, and often big enough to completely fill the hallways you're supposed to be sneaking down. To make it all worse, you get no rewards from killing anything in the Breeding Zone. No exp, no gold, nothing.
Tier-Induced Scrappy: In MMVII, Clerics are the only class that can cast grandmaster protection from magic, which is the only defense against monsters which can automatically kill or eradicate players by hitting them. This makes the Cleric more or less essential, and an annoyance to players who like playing different classes for variety and would like to do 4 different class playthroughs rather than 3 different classes + cleric playthroughs.
Unwinnable by Insanity: It is possible in Might and Magic VII to make the game unwinnable during the Warrior-Mage promotion quest. The quest is to replace a pulley belt on the elevator mechanism in an abandoned mine with a weak belt that's meant to break after an hour, rendering the elevator useless (and you unable to leave the mine if you're still on the lower level at that time). You can still get out of the mine by having the party wiped out by the many medusas and oozes in the lower level... unless you've defeated them all, and aren't lucky enough to have a teleport spell on your person.
Actually, it's really not that simple. In that game, the two teleport spells - Town Portal and Lloyd's Beacon - have some conditions. Town Portal can't be cast with hostile monsters in the area, unless the caster is a Grandmaster of Water (and likely isn't at that point) so if that is to be an option, defeating all the monsters is likely required (and in this dungeon, they're tough). Lloyd's Beacon can't be cast at all unless you're a Grandmaster. Basically, unless you do wipe out all the monsters, making a run for it (maybe with Haste) is the best idea, after saving the game first.