Fives absolutely can be obnoxious to go up against. High health, calls assists, counterattacks, and debuffs. Where's the weakness? Of course the main solution for him is stunning, but then again that's true of...every other character in the game. Fives has no unique hard counters (unlike, say, RG and dispels) and is himself a counter to, uh, pretty much everyone.
IMO the combination of counterattacking and high health is a bad one. People don't not attack Fives early because he sucks (ala Old Ben), they leave him alone because the counterattack will murder you early on and it's often better to go after a squishier toon first to get one out of the way early. The problem, of course, is that leaving Fives til last can also be a bad strategy, because by then your toons will likely be hurting and his counterattacking can wipe out multiple people.
Compare Fives to Oppress - the latter is also a toon I generally leave for last (or nearly last). Same high health but no assist calling, no counterattacking, and so-so debuffing. He does get the big damage buff when you hit him, but that's dispellable and much more easily controlled. If you have a low-health toon who doesn't have any non-attacking abilities off cooldown (like a heal or buff), you have no choice but to hit Fives and lose your toon. Oppress I find pretty well-balanced; Fives I don't.
Tanks are balanced because they don't do much damage, but can control the battle and take a long time to kill. Healers are balanced because they also don't do much damage (though Lumi is pretty good) but can prevent others from dying. Glass cannon DPS are balanced because they will punish you if left alive, but die easily.
Fives falls into none of those categories. He is a tank and ignoreable...right up until the point you start attacking him. Then he instantly turns into a high-DPS, high-health toon. Where is the balance in that?
I think it's fine leaving him as a tankier DPS, but his damage needs a significant nerf. The easiest solution would simply be to make his counterattack only function below 50% health. Alternatively his abilities could be left the same but have his health nerfed, but I think that would make him too similar to other toons.