I'm not quite sure, I don't think so, the main use for me is the ridiculous hitbox it has since it ends many early fight very fast, the stun is the cherry on top.
Flare are pretty neat since they can room clear some places on their own but I never use pepper spray (to be fair I don't even buy tools at all so I don't get many to begin with)
I try to not rely on criticals unless I'm using the P08/P229, but yeah with my upgrade route I'm lvl 1 by N°09 and lvl 2 or 3 on the Flamer boss so just in, time to not have to worry too much about it
The M203 is neat on paper with riot grenades but I end up being annoyed by it more than anything else since the hammer does more or less the same thing, Hammer is godlike against anything that can be interrupted (aka chasers and blade golems) by it so you can just stand and dump your mag into things, sure no crits but things die fast all the same, Pyke you just save 17 seconds of it for the one boss and use the rest on the smaller underwater things for situations where Firefly ammo aren't necessary.
As long as I have it by N°09 in dryfield it's fine, the rest you have more than enough firepower to deal without it, I don't even use it outside of multi Golem fights and bosses and maybe a few of the larger fights.
Aren't the earth PEs much better for the MP pool?
Metabolism is kinda too slow to be useful at lvl 1, especially in those latter modes where if you get hit by those you're already most likely fucked anyways, the status prevention is also a bit too short for my tastes.
Scavengers I tend to just use Flare/Hammer, Chaser I go plasma or hammer + anything that hits hard (Pyrokinesis, Bayonet, P08 if I'm not in any real danger, M950 even sometimes), Stalkers are a joke just get in a cramped space and magdump your P08 w/Snail mag, it rarely ever fails surprisingly, you can even Plasma them and if you're lvl2 or better it's basically gonna be refunded MPs.
Good MP management overall in any mode (taking into consideration you get a fresh refill everytime you level a PE) makes the game considerably easier since even in Scavenger you can really spam PEs all over the place.
I actually never knew about half of that (attenuation especially but the crit chances on M4 with different attachments also), I never even used a guide until after my 20+ playthrough of the game I also never knew about the super secret weapon seller right before the final bosses until that, there's plenty of shit you don't exactly know but are lifesavers, the free Javelin/M249/Moongoose with the Medicine wheel for example.