the power curve of the character, how fast monsters become too weak / too strong to be usable. 2 ratings below and they are cannon fodder, two rating above the players and they wipe the floor with them.
That (or rather avoiding it) is the whole point of bounded accuracy. Your proficiency bonus does not increase as fast, you are not gaining all these incremental bonuses as you level, etc.
There is much more to PF2 encounter math vs 5e encounter math than just using CR vs Level as the unit
oh, you're referring to how numbers scale? i mean, okay, except 3(.5)e exists. the numbers in 3(.5)e scale at least as fast as they do in 4e/pf2e, yet the CR system there is about as good as 5e's (which is to say, it's not).
also, pf2e has a proficiency without level option which brings the number scaling pretty close to 5e, and while it does change how encounter building works pretty drastically, from what i've seen it doesn't exactly break the entire thing open (unless you go into the extremes, which the rule warns about). and yeah, of course stuff there is wonky, because the game was designed to add your level to your proficiency bonus. that doesn't automatically mean you can't make a functional level based encounter system without that kind of scaling.
but, i mean, yeah, those games are designed around using level as the unit of measurement for how strong a monster is...and they're better for it. that's the point. maybe you can't really do it in 5e, but it'd still be interesting to try.
i'll be fair here, though - a5e still uses CR (because it wants to keep compatibility with 5e), and from what i've heard the changes they make to CR and the encounter building system make it actually pretty decent. so i guess it's not like you
can't make a functional CR system, but rather that it's probably more difficult (given that only 1 game i know of has done it, as opposed to at least 3 for level).