I know how to play with any/all of the weapons in this game. I can still do well with them but will not be killing things quite as quickly or effectively.
Don't take this the wrong way, but there's no way that's true.
Pistols - Arc Pistol and Talon can go on literally any kit on the game and destroy. Scorpion is my default secondary to deal with phantoms if I don't have powers or a primary gun that can do the job. Acolyte is extremely niche. It's the best shield stripping gun, but doesn't do much else with the effectiveness of the other pistols.
Shotguns - This is probably the best class of weapons, meaning there isn't the gap between best and worst. And piranha is freaking amazing, especially with it's ammo, rat e of fire, and weight.
Snipers - Widow is amazing. CSR is amazing, Kishock is amazing. Heck, even the Mantis does really good damage for it being a common gun.
SMGs - CSMG is amazing. I use it more than the Hurricane because of it's ammo and accuracy.
There are plenty of good guns that I use quite often. I never feel cornered into using only a few select weapons to do well in this game.