Being an Actionized Sequel to its predecessor makes sense for the same reason it did in Resident Evil 4. Jill already has experience fighting zombies after that fateful night in the Mansion Lab, and has likewise been mentally preparing herself for a second round with Umbrella's horrors just in case. Even if she hadn't, she is a veteran police officer with a military background (as a member of a glorified SWAT Team and Delta Force respectively), and thus would be both more physically and mentally prepared to handle such a situation than Rookie Leon or Civilian Claire.
When Carlos saves Jill at the beginning of the game, he does so by blasting Nemesis with a rocket launcher, which is only enough to incapacitate him. However, it took two rockets, since Nemesis caught the first one and tossed it away. He could have easily done so for the second shot, yet chose not to. Why? It's already established that Nemesis is scarily intelligent, and likely wanted to lure Jill into a false sense of security so he could get the drop on her later on.
When Jill escapes a rocket launcher wielding Nemesis, Mikhail gives new orders to Carlos and Tyrell. They are supposed to rescue doctor Bard. While Jill, Mikhail and Nicholai are transporting survivors out of the city via a train. Nicholai is then revealed as a traitor, when Nemesis attacks the train. Why did Mikhail let him on the train in the first place? Why did he not send him to find Bard instead of sending Carlos? Because he already knew Nicholai was a double-crosser. He probably figured that Nicholai would only kill Bard or wouldn't care of his order at all. He sent people whom he trusted (Carlos and Tyrell) and kept the dodgy Nicholai by his side so he could have a control above his actions. Then Nemesis appeared...
In a situation like Raccoon City's in RE 2 and 3, your resources are extremely limited. That includes human resources. Mikhail may have mistrusted Nicholai, and we as outside observers know that he'd ultimately have been right to do so, but having one's superior officer kick out or outright kill someone on their own team is a heavy cost regardless of whether they're right to do so, especially in the middle of a zombie emergency with a train full of civilians to evacuate. Mikhail may not have had the damning proof to do so, and had he turned out to be wrong it would have jeopardized everyone right then and there out total paranoia and breakdown in team cohesion. Judging from Carlos' initial reaction to Jill's comments in their first meeting UBCS is a branch kept separate by Umbrella itself from its other going-ons, so there's no reason to believe UBCS is too different from other military-type first-responders, where the team is key and relying on your teammates is paramount for a successful mission. Going against that kind of training, even for legitimate reasons, is a difficult mental block for most to overcome, especially in a stressful situation.
Carlos has a much harder time fighting the Hunters Beta because his assault rifle lacks the hard-hitting knockdown power of Jill's shotgun, magnum and grenade launcher. Of course, Hunters are B.O.W.s, and the W stands for "weapon", meaning they're designed to be deployed against military targets. As it happens, most infantry are carrying assault rifles similar to his (okay, it's true that calibers like .223 and 7.62 so pack a lot more punch in real life than they do in-game, but we're playing by the setting's rules, here).
Fridge Horror
When the game starts, Jill's having recurring nightmares from her PTSD of the Arklay Incident, which seems to be seeping into making it hard for her to sleep. Then you can check the bedside after she's fully woken up and see she's practically tearing into packs of sleeping pills, even regretting how many she's probably dosed herself on — or overdosed, which clearly aren't working like they should. So not only is she having to survive an apocalyptic outbreak with a nightmarish abomination of human science out for her head on top of all the zombies and mutated creatures running amok, she's pushing raw adrenaline through a sleep-deprived body heavily medicated on pills, likely dulling her reflexes and reactions more than they should be, and she's fully aware of the unforeseeable folly. Though at the same time, this just leads credentials to her Action Girl capability, as she's effectively handicapped and still escapes in one piece as one of the most mobile and effective survivors of the incident.
Leon and Claire's run through the west wing of the RPD station features relatively few enemies until reaching the library. In RE3, that wing is chock full of enemies, to the extent that Carlos, who has plenty of ammo and an fully-automatic rifle, is very nearly overwhelmed. This is most clear in the shower area, where Leon or Claire would've almost certainly been killed had Carlos not cleaned house as he did.
What's really disturbing is that the Lickers probably ate the bodies that Carlos left lying around the building, and most of them seem to be infesting the RPD's vents. If Leon or Claire had ever needed to look into the RPD's ventilation system, just imagine what they'd have looked like by then.
After Marvin gets bitten from Brad, he runs away from Carlos and Tyrell in confusion, or out of fear. Headscratchers covered, he might have run to the east wing of the RPD, which may still house 'survivors' there. If Carlos and Tyrell hadn't been on their own mission, they might have tried to rescue a considerable amount of people. But the east wing in Resident Evil 2 (Remake) is now just a hallway and and office filled with zombies.
