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.
Heretic II introduces the harpies which are even worse, since they like to hang back very high in the sky, dodge just about everything you can throw at them. Unless you use the homing meteor swarm on them.
I Am Not Shazam: Some of the old FAQs name the protagonist Heretic, since he didn't have a proper name at that time. Funnily enough, just adding "the" in front of it would have made it appropriate...
That One Attack: All the Iron Lich's attacks hit like a freight train, but are fairly easy to dodge...except for the tornado, which chases down the player, lifts them up and flings them around (and can easy hurl the player into a damaging floor if there are any around,) can easily last a very, very long time, and in certain scenarios can absolutely ravage the player's health. The only saving grace is that they get fairly easily hung up on the architecture.
That One Level: Catafalque (E4M1) follows in the tradition of the first level of Thy Flesh Consumed from Ultimate Doom by being a Drought Level of Doom that also tosses in a boss-level enemy (in this case a Maulotaur) into fray and forces you to fight in tight spaces. Except that Heretic's level is even more stingy with it's pickups, and also throws in four Iron Liches as an extra "Fuck you." Better learn how to kill things with the staff and gauntlets.