We have seen significant client drops and failure to re-associate due to APs with less than 5 clients reporting a "ap is resource constrained" issue. Our current firmware is 6.5.4.5. I disabled Band Steering, Client Match, Client Aware, and Scanning.
I re-enabled Band Steering, with no issues. When I enabled Client Match, the APs began reporting the issue again, and clients were having problems connecting. I have an open TAC case, and they are currently trying to replicate the issue. The current recommended solution is to upgrade to 6.5.4.7, but the release notes do not mention these issues.
Still waiting on a permanent solution. Managing 10+ year old Motorola/Zebra devices without help from Band Steering and Client Match is not a fun time.