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.
With the release of 2.1, the face-off against Metal Sonic in ERZ3 is this.
Broken Base: Not everyone will agree with some of the major changes 2.1 brought into the table, with the most relevant victims being the level revamps and the new NIGHTS-styled Special Stages.
Some also dislike the new Knuckles, dubbed "Chuckles" due to his idle animation being the mischievous laugh from Sonic 3 instead of him punching in place. Also considering the clash of styles between the current Knuckles and the older Sonic and Tails skins.
Tails can easily fly through irritating bottomless pit sections... But even he has trouble in Egg Rock Zone.
Knuckles's gliding and climbing abilities can similarly be used to bypass a lot of platforming sections. It's actually worse with him, since the levels are rarely, if ever, designed against him, and there's no timer for either of his abilities.
Goddamned Bats: Sharps. They are blue hockey-puck things that turn red with a loud noise when they see you, and then they chase after you at very fast speeds while invincible, and the whole "get hurt and attack with temporary invincibility" thing won't work. They turn blue again after a while, but you're probably still collecting your spilled rings.
The Buzz enemies from Techno Hill Zone are also fairly annoying, with their tendencies to pop up out of nowhere and knock you off platforms.
Jetty-syns are infamous for their ability to harass players with their unpredictable flying patterns and tendency to attack without warning. Their appearances have been reduced with each update, to the point where they now only appear in Egg Rock Act 2 and Aerial Garden.
Goddamned Boss: Techno Hill Zone's boss. He flies around in circles, splattering around a lot of harmful goo that takes a very long time to disappear. If you don't defeat him quickly, you're going to have a hard time stepping around the gunk. It's also difficult to hit him because he may suddenly change directions as you jump.
What makes him a Goddamned Boss instead of That One Boss is that once you calm down and figure him out, you can easily kill him by standing still in the right place and jumping.
Justified, in that it's supposed to be preceded by two more acts for pre-existing zones, another zone, and a possibly one-act zone. As it stands, the absence of those four zones makes it a jarring Difficulty Spike. The game is still in development, but priorities shifting to the multiplayer aspect put single player stages on low priority.
But even after that, ERZ will still be the hardest level in the game, unless it somehow turns into a Breather Level...
Word of God has stated that ERZ is supposed to be that hard.