One certain downside of the mirror is that it blocks the viewfinder when you're shooting video.
And some DSLRs have 12 minute limits (e.g., Canon T2i/D550; T3i/60D; 60D; 7D; 5D Mark II) some have 20 minute limits (early Nikons) and some have 30 minutes (Canon EOS M, 70D, 5D Mark III, modern...