I’ve seen the following bad things happen if a hive is too warm in the winter:
1.) More bees will attempt to fly in the snow and in sub freezing temps. Bees seem to lose their sense of orientation in the snow. If they land in the snow, it cools off their bodies and they’re goners. Yes, you naturally lose bees over winter no matter what, but I have seen many younger bees die in the snow as they crawled out of a hive that was too warm. You need those young bees in the spring if you want a strong buildup.
2.) I’ve had cases where the bees have completely broken cluster in the middle of winter and become so excited they heated the hive to 90F. The hives were buzzing and acting just like summer hives and consuming more honey than their cooler/normal neighbors. In my cases, those bees croaked unless I was able to get them cooled down and back into cluster.
3.) If the hive gets too warm, you risk the pests starting to reproduce. In my area, I only have to contend with wax moths. If a hive is warm enough, the wax moths will multiply even in the winter. The bees don’t seem to patrol the comb as diligently in the winter as they do in the summer, so you can end up with a hive full of moths come spring.
Don’t get me wrong, I am in favor of warm hives during winter; but not hot ones. The bees do great in a warm hive whereas they croak in a hot hive and croak in a bitterly cold hive in my climate in my bee yard.