So I'm trying my best cutting dovetails in 12mm (~1/2") shedua.

I remove most of the waste with my fret saw so there's only 1-2mm waste left for the chisel.

Using a 10mm E.A. Berg chisel and a small chisel hammer I make very light taps on the chisel to control the cut to remove the remaining waste in small increments, i.e. no one-time-heavy-cut that I would expect to damage the chisel quickly.
After just two or three cuts, the bevel is rolled over and the chisel is dull and useless.

I've sharpened the chisel using the Scary Sharp method, i.e. very very fine self-adhesive sandpaper fixed to a glass plate.
The back is flat and the single bevel is 25 degrees, both with a mirror finish and ensuring that any burr is removed completely.
The chisel is sharp enough to shave hairs from my arm if that tells anything....

So, is the chisel poor quality? Am I sharpening it incorrectly? Am I using the chisel incorrectly?