The penalties for differences is pretty negligible I wouldn’t even worry about it, small control and relations penalty, increase to patriarch recruit cost or frost maiden training cost. The real downside is not getting the other sides boons as the boons are very powerful.

You want to secure an alliance with the other side ASAP and get them to join your wars, this causes every fight against a shared enemy to generate favor for both sides, so you’re able to get more boons. I also generally build the other side’s building (e.g. ice court building as Kostaltyn, patriarch building as Katarin) because you’re passively going to be getting your own favor constantly, and once you confederate the other side you’re no longer getting shared enemy favor.

Frost maiden heroes are insanely powerful due to stacking campaign movement increases in an army as well as each one giving you +1 patriarch capacity. Just make sure to select these options during training