In the case that I gave as an example - pet eye AF - this is about the resolution and coverage of the AF sensor. With on sensor AF you can have thousands of AF points spread over nearly all sensor. In a DLSR you have tens, typically low tens and in basic DSLRs like 200D you have 9 all bunched up in the centre. Tracking a small eye of a moving pet requires a fine granularity of the AF system and wide AF coverage. Not possible on a DSLR in a regular mode i.e. when using OVF.The lack of a mirror does not make code development and integration easier. Using it may require one addition step, mirror lockup, which most every recent mirrored camera likely already has available.