I second Coven units. 8 Grots backed up by some CC Characters is the solution to many problems. Peoples main error when trying Coven is not doubling down hard enough.
As you said about the Nids, Coven have a tendency to "overload" the enemy with threats. Grots have basically the same profile as your transports (T5, 4++, 6+++), but are great in CC where your transports suck.
You just need to position well (and have some secondary screens/screening raiders/etc).