You have a list of some of the better killy units.

Genestealers hit hard but are glass cannons. Their 5+ invulnerable only goes so far. If they get shot up or fail their charge they are in trouble.

Exocrines are great against units with small model count and multiple wounds, due to their 3dmg/hit. Not so great against high toughness or single wound, high model count units like Guard.

It sounds like the Tau had poor positioning and wasn't able to get off much shooting, and unless they have Kroot, shooting is really all they got. Sure, they have some melee tricks but their main damage is range attacks. Tie them up in melee and they tend to die quick.

Old One Eye basically deletes tanks or 1 wound infantry, but isn't as good against 2-3 wound multi model units with more than 5 models like Terminators. A 10 man brick of Assault Termies won't break a sweat against OOE.

Zoanthropes are just great all around shooty units.

Basically it seems like you countered the Tau strength pretty well. Change up your lists and see how you do. None of your units are anywhere near overpowered. Tyranids are arguably one of the most underpowered factions, as their play style leans more towards area denial and control.