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.
Anticlimax Boss: Blanchov. After taking out his Dragon (in a much more epic gun battle), a cutscene activates showing Jack casually walking up to Blanchov and shooting him in the chest just once. After that, he dies. You don't even have to touch the controller.
You fight Diggs in a gas chamber. As the fight progresses, the gas slowly drains your health; you and Diggs take turns punching the mask off each other's faces and stealing it for air, which draws out a fight already stretched by Diggs' endurance.
Fahook is fought in the back of a plane with limited cover. His gun will shred you to pieces if you're not careful, and you have to use the bullet time dive and the right weapons with precise timing in order to hit him before he hits you. Occasionally, Fahook will take a swig of a magic potion and start breathing health-bar-destroying fire at you (yes, really), at which point you have a limited window in which you have to shoot the potion out of his hands. To make matters worse, towards the end of the fight the back of the plane opens up, and you can get sucked out yourself if you aren't careful.
Hennessey is even worse. He starts the first stage of the fight with an electrified riot shield which can kill you in a few hits, and will constantly charge at you, so you have to try and lure him into the water to short out his shield without getting hit yourself. After a few rounds of this, you have to fight off some more GAC squad mooks to reach the final melee fight with Hennessey. Hennessey has ridiculous endurance and will annihilate you in seconds if you haven't mastered your melee attacks. Fight back for a while and Hennessey opens up the door to the room's incinerator, then resumes his melee onslaught with his fists on fire. And at his last sliver of health, your melee-weary hands better be ready for a quick use of reflexes, because you'll have to knock him into the incinerator. Did we forget to mention there are no health pickups whatsoever during this fight?
The helicopter in the second game is also a real pain in the ass.
Strike that, the entirety of the prison level, where you have to grind several different minigames to get enough cigarettes to bribe a guy for some stuff to escape prison. Later adaptations of the game made this skippable, at least.
Sure the prison level was a bit tedious, but it's NOTHING compared to the harbor level, which has one section with several snipers were you will very likely die dozens of times as there's virtually no cover and it's damn hard to find a position where you can kill the snipers without being shot yourself, and god help you if you wasted too much ammo in the beginning of the level, because then that section becomes a Luck-Based Mission.
I cast my vote on the return to the Construction Site. The fact that you wind up having to fight a chopper (again) doesn't help its case.
They Wasted A Perfectly Good Character: Patch. He's a suave assassin who has a fun character quick and a nice British accent, complete with memorable introduction. He does not have a single line after that, and proceeds to die as anticlimactically as possible.