The issue I have with the Mimicry Marcus's is that most of them can be avoided. The little action they do when they spot you can give you just enough time to run to the next door. There are only 3 or 4 that I can think of that can really pose a threat (all of which are near the end of the game), and dealing with them can create a bigger problem with their explosion and the resulting leeches. With Code Veronica, the Hunters, Moth, Nosferatu, and the Tyrant are all pains to deal with. Then if you aren't prepared, Steve's section will always kill you unless you run around him a certain way, and no casual player will know to try that. The puzzles in 0 aren't very tough either, though to be fair Code Veronica didn't have any that were extremely difficult from what I remember.
Regarding 2 and 3, I try not to consider unlockables or glitches because those aren't the ideal way to experience the game. Looking at it from the base game is what I like to do because that's what sticked with me more when I played it.
In regards to mimicry marcuses, ı think ı defeat them all except the one in one of the rooms of training center which leads to bathroom ( Another one will also spawn if you explore this further and this can be avoided too. ) and the area where you obtain mo disc by inserting both microfilms into the projector. The other ones ı usually defeat but ı remember avoiding more in the past as well. I still think they are difficult to even evade. They are meant to be defeated with flame based weapons since this will prevent them from doing the explosion attack due to them burning to death in the process. Only molotovs or grenade launcher loaded with flame rounds can achieve this.
For the game's puzzles, ı find them difficult to the point ı made a guide by myself to solve them easier during my runs. The most difficult one is the one with lighter where you need to burn animal statues in correct order to open a path. The box pushing one is difficult as well.
As for recv;
The hunters arent that difficult since most can be dispatched easily with smgs or shotgun. I think the only difficult hunters are the 2 ones which spawn at the area where gulp worm 1st spawns during claire's story.
Moths are more annoying than being difficult, you're meant to avoid them rather than fighting. In case of poison, there's an infinite blue herb plant in the room.
Nosferatu is difficult so ı agree with that. But there are special ways to defeat it so if your timing is good, it can simply be defeated with the knife which features a special kill animation. I dont find sniper rifle that good since aiming is awkward for it.
T-078 isnt that difficult either since while in plane; it can be dealt quickly with simply 3 bow gas rounds and 2 acid rounds. And the previous fight is even easier.
For steve, you can safely run away from him with enough health items. You only need 2.
For the puzzles, ı dont remember them being that hard in the game.