**post contains spoilers of unlockable weapons
I'm assuming this is your second play-through. (You mention the two handguns/reapers that are only unlocked once you finish the game once.) While the 2nd play-through weapons are kinda neat, they're not necessary to get you through the game. Again. (If you've stepped it up to difficult, I mean.) The reapers, in particular, lost their usefulness to me pretty quickly. The only unlockable weapon I found myself using all the time the second time through is the flamethrower. (L11-2 Dragon) With this awesome unlockable you can basically run through hallways and small rooms of enemies with sprint-like ease!
Having said that, though, once you get the hang of things, all you actually need to get yourself comfortably through the game is the carbine, bullseye, rocket launcher, the sniper gun, and maybe an air-grenade or two. (Plus the regular grenades.) Uncomfortably, all you really need is the bullseye.
As for the other unlockable weapons that you may have missed, I'm not sure I'd recommend going back to get them. Actually, I wouldn't...unless you're excited about novelty of them. There's a backlash grenade that's ok, but I never found it more damaging than a hedgehog or frag grenade. (It creates a giant bubble shield around an enemy that reflect all his weapon fire back at himself.) The splitter gun is...well....weird. It shoots out big blocks of...some kind of funny-looking ammo...that you can split into three blocks if you want. Apparently it's pretty effective against Titans, but as much as I try to use it in regular battle, it's just too...clunky and gimmicky to me. Albeit powerful. Same goes for the arc charger. Yes, it's powerful too, but I found it hit or miss trying to bounce the charge off more than one enemy.
Again, all I'd recommend is the flamethrower of all the unlockable weapons. And you can find that in the Bristol level, "Devil at the Door". (Which is well into the game.)
Good luck with your game!