they refuse to use the counter to said "OP" thing.
"HURR DURR Dual Band/TF/MMS are all OP!" Cold Blooded. Use it.
"DERP Concussion and Flash grenades are OP!" Tactical Mask is your friend.
"OMIGAWD C4 is sooo OP!" What is Flak Jacket?
It's not rocket science. Set up your classes accordingly. You're given more than one class for that reason. It's not like the game is forcing you to run lolhardwired or lolblindeye. I have no sympathy for people that complain about certain things in the game when they refuse to use the perks that counter those things.
Well cold-blooded doesn't really counter Target Finder. If it was never in existence and we just had the Dual Band for everything, then cold-blooded would be a hard counter, an actual reason to not use the sight. The target finder is at worst a better sight attachment than reflex, acog, or hybrid.
And Flak does counter C4's lethality, but there's nothing that counters the fact that C4 is the strongest, most versatile, and most accurate lethal (Accuracy here meaning kills/uses). Yeah Flak would be the solution if the problem was explosives, but the problem is that one explosive blows the others out of the water.
I think this game puts a lot of pressure on solo players to use Hard Wired and Blind Eye. Not that there's anything wrong with those perks. Flak Jacket and Toughness aren't as necessary as some may think.
C4 is only so powerful because most people want to use Lightweight, Hardline or Ghost. Which means two things: yes if you complain about it but you don't use Flak then you're a self punishing fool. It also means the people who do use it so often are the kind of players who base their strategies on the shortcomings of their opponents. Upon making contact with a functional, deceptive opponent these C4 spammers will quit just as fast as their victims do.