Finished SS3 on serious difficulty on Fusion just the other day and there are some things I didn't like about it but man, overall it's still an excellent game.
A lot of people shit on the game's pacing because at first it really doesn't feel like a Sam game: the enemy numbers are just not there, the level design feels like a middle ground between Sam and Duke 3D with all the realism and the bit more complex layouts etc. but in the grand scheme of things I think it works wonderfully. SS3 has this gradual build up and because the first half of the game is conservative with the enemy numbers the second half's large battles can surprise you. It also provides a wonderful bit of "environmental storytelling" with how the game starts you off in a city with Sam and a bunch of other characters and how gradually everybody and literally every sign of modern civilization disappears, leaving Sam alone to witness the Apocalypse. You're playing this cheesy game with the badass protagonist but by the end you do feel like you're the last man on Earth and in the back of your head you're like "wow, this was the last day of humanity".
Fusion's new additions took some getting used to but overall they made the game more interesting, I liked the new secondary attacks the enemies have here. Croteam optimized the game's performance a lot too, I never dipped below 60 fps on ultra settings and that's wonderful compared to the original game. SS3 could be a bit too much for DX9 and a single CPU thread but those issues are non-existent now, I saw a comparison benchmark and in some places they managed to double the framerate on identical settings.
There are a few things I found bad though and while Fusion improves on some of that I don't think it will be able to remedy design decisions that are just bad. First: the underground levels are super boring and they are not fun to play. At those parts you get a flashlight to view dark areas but your view distance is still limited so exploration doesn't feel right + Croteam slows down your movement speed guess because you're supposed to take in the "atmosphere". Bad idea and the special enemy type you encounter underground, the creatures Sam calls "space monkeys" are boring to fight against.
Second: even though Fusion makes hitscan enemies more bearable by making them less accurate if you're moving overall there are still just too many of them. It's not fun to shoot 50 of those hard to spot black soldiers because the fact that you have to creep around corners really slows everything down. Hitscan has a place in Serious Sam but the yellow and red scorpions were perfectly enough and balanced, instead of that duo you still get the red ones but the yellow ones are nowhere to be seen and replaced by a smaller, weaker variant. Why? Because if there is a weaker scorpion then the game can throw a lot more of them at you = more hitscan on top of the new black soldiers. Fuck that, when those two are around in great numbers you have a ruined combat encounter.
I don't get the witch brides either: they work as this interesting yet mildly annoying enemy type when you only have to face one at a time but on the last level you're attacked by at least 6 of them at once (while around two dozen small hitscan scorpions are on your ass... yeah, it's absolutely the worst part of the game). That shit just doesn't work, Croteam!