The problem is that you want to be able to migrate processes between cores. It's not the end of the world if a number-crunching process gets stuck on the big core. But if a process detects an instruction that it's only using for convenience purposes, and that causes it to get stuck on the big core or stuck on the little cores, that's not good. In a different scenario, if it was set up so that a process starting on a little core never detected an important instruction, and was stuck in slow mode forever, that would also not be good.

It depends a lot on which instructions differ and how the feature flags work, of course. They might have set it up in a way that's fine.