I had my first defeat last night. Attacking Earth, I think it had 16 population, 100% approval, 10% defense (I don't recall exactly). I had a 5% chance of victory with a 4 pop transport. The second transport succeeded. So even though I know it isn't strictly speaking bugged, it seems off-balance.
I guess I haven't seen such an extreme case as that yet, but it is pretty crazy that it takes that much to resist a single transport. So much so that it more or less isn't worth trying. I don't bother building any defense any more because it isn't a worthwhile investment.
The attacker always wins. Always. No matter what. I've seen battle as lopsided as attacker power of ~2 vs a defender power of ~9 and attacker wins. It seems like something is wrong.