It's is compatible, it's not recommended to use them in the array, but you still can, just won't have trim support, no issues when using them in a pool.
Depends on what you compare, if you mean the array that should be obvious since data is not stripped, Unraid pools can be as fast as Freenas, I get 1GB/s transferring to some of my pools.
That suggests a hardware problem, obviously most users don't have that issue or the forum would be littered with complains.