Mirroless systems can only use Contrast AF which is slower than Phase Detection AF, much slower in lower light because there is less contrast, can cause some focus hunting, and can't track focus. So while you may have a high fps camera the focus system can't keep up if the subject is moving much for example in sports, auto racing, kids running around, etc. etc. etc.... you know, all the things high fps is most useful for. However, the Contrast AF algorithms might improve in time or some other AF system might be developed but it's not on the horizon. Mirrors are not going away any time soon.