I have both a D800 and a7s. While I like the size and photo quality of my a7s, the auto focus just isn't good enough (for me) to replace a dslr. The quality is there, but I wouldn't rely on it alone unless for myself (vacation, travel etc). Also for use with strobes/flashes, there are fewer options (at least with ttl) so if that matters to you, it can be an issue. I just got an indra 500 ttl strobe and it has a remote trigger for Nikon that works amazing, but I don't think it comes in Sony flavor (only option with hss would be the mitros+ maybe, but no reviews to judge how well it works) and other systems (Fuji, m43 etc) I don't think I've ever seen any serious lighting options for that. Mirror less will probably be the future, but for me personally it's not there yet. For lighting, you'll need manual only (which is fine and in a lot of cases preferred) but in the case of Sony, I hate the proprietary hot shoe that makes it so you can't push the flash in all the way. I haven't seen a mirror less with dual card slots. Not a deal breaker but a disadvantage. One more disadvantage I experienced is controls. At least in Sony. Moving focus point requires too many button presses though m43 was awesome with touch to focus screen which could move the focal point and shoot if desired by just touching what you wanted focused.
Advantages? It's really nice to see the way your exposure in real time. The weight is nice (though a7s maybe not the best example since those full frame lenses still have a bit of weight) especially m43. When I had an Olympus em10, it was very easy to carry around. Ibis is a fantastic feature.
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