So, after losing an auto-resolve to a creature with 1/3rd my combat rating (8 to my 27), I thought the issue was with the auto-resolve. Then I fought a battle and lost in 2 rounds, without doing any damage, to a sovereign + 1 minion that, again, had a lower combat rating than me.
It went something like this: my sovereign missed 2 spells in a row (no idea why). Then the enemy's minion cast a spell that damaged everyone; killing 2 of my 4 people and knocking the other two to almost none. It was then easy work for them.
So, aside from the already-complained-about spell missing (and there's still no indication about why that happens, or if it'll happen before hand), combat rating is not an accurate predictor of relative strengths. Well, to be precise, it's unclear if the other side's is lower. If theirs is higher, it's a pretty safe bet they'll stomp you into the ground. But again, it could be lower and they could still do the same thing.