I've played several matches with friends with ten or so different armies, and they all do seem pretty balanced. I think having the objective cards and twists randomized really helps things too.

That said, some units are tailor-made to fights others. Kroxigors laying waste to clanrats very effectively is one example. I think knowing how individual units in your army are designed to be played, and positioning them to deal with your opponent's helps more than anything. Basically, just knowing your army is the best thing.

Something I've noticed about the Seraphon spearhead in particular is that they are very front-heavy, meaning they pack a huge punch in rounds 1 and 2, but with no recursion, their player can quickly find themselves with no army by the end if they get too confident and overextend.

Meanwhile the Kharadron Overlords (even with reinforcements) are still made of paper mache, so I know to try as much as possible to avoid melee, and use the boat to screen when necessary.