About 11~12 years ago I made some mirror frames (the biggest was 32" x 48")- mitered corners and floating tenons - still no problems...

About one year ago, I made a frame for a glass table (26" x 26") and re-enforced the corners (actually, the floating tenon) with two small nails on the back side as on the pic below...(you can screw a 90° iron on the corners).

Regards
niki