I have done heating the room instead of individual cages. Right now, that's how it is for mine. You do still end up withs ome gradiant, unless you have a super well built room and good airflow. Since my cages back up against the wall, the backs tend to be higher or lower temps(according to winter/summer) and the fronts are the room temps. I spot-check temps often. The lower bins tend to be cooler, with the upper ones being hotter, but a fan to circulate the air helps keep that to a minimum.

It can work fine, if you are careful to check temps a lot.