It's makes the worse case sufficiently bad and unfair such that it makes things worse overall. If the lock is contended by a thread with higher priority, then that blocking thread will have its priority increased. Now if the ends thread to get the lock is one spinning on it rather than actual high priority one, then this will repeat, leading to large latency on front of the high priority thread and a lot of misaligned CPU utilization by a lower priority thread.

Spinning on a CAS is far more expensive than spinning on most other instructions as well as it affects all core that may try to access that cache line, which may include things other than the lock itself.

Also consider how the system acts under high CPU load. You will end up with threads holding locks when not running leading to the majority of the time you miss the lock you spin all 100 times. This just exacerbate the CPU load issues even more. Hybrid locks are only helpful under lower CPU load.