It's really hard for me to say.
I'll be using Metro 2033 for this example, but you could substitute Max Payne 3 in and it's similar.
I usually don't have much trouble with the human enemies, pretty much steamrolling it on normal. Though occasionally I'll get in a weird spot, usually where enemies are directly above me, and get dropped in half a second(i guess due to headshots). In most cases there has been a way to get up to their level, but there was on place in particular where you're on the surface about 5 seconds and you run directly into like 20 freaking nazis or whatever they are and they're all over the damn place above you and there's no way up except through them. I spent about two hours on this one spot before giving up and turning it down to easy, then another 30 minutes to actually move on.
It's made extra hilarious because said nazis wear helmets that provide 100% protection against head shots and take like 2-5 shots to knock off. The gas masks they have to wear to survive are a weaker target and ensure they'll die(2-3 shots), but they're very hard to hit and actually have it register on the mask and not helmet.
Tbh it seems to me like going from steamrolling on normal to barely being able to progress on easy, then going right back to steamrolling on normal again would be a design issue(headshots, in this case). Obviously on harder difficulties, you would want to leave it in, but it just seems really weird on easier ones.