I think you know the answer anyway. A mitred joint is the weakest possible joint as it relies solely on the glue on two facing surfaces. Which is why it is underpinned or splined or biscuited - as these give additional surface area for the glue to attach. A sliding dovetail, on the other hand, is a mechanical solution, the glue being just a bonus.
I'm not quite sure what you mean though. I can't quite see how a sliding dovetail joint fits into a normal picture frame?
Unless this is for the flat part of a cassetta* frame - with mitred inner and outer bits attached? If that's what you mean, and you've a workshop and a decent router, then why wouldn't you. Probably takes less time than putting a spline or biscuit in. I guess this could well have been made that way:
Or maybe if making a Nicholson Frame (and I mean a Nicholson Frame, not a bog-standard cassetta 'St Ives' frame). Or perhaps for a sub-frame.
This is a cool idea for using a biscuit cutter to make splined joints: http://www.woodmagazine.com/woodworking ... rced-miter
*I see you've a couple of these on Instagram. BTW the link to Instagram from your website doesn't work.