Ronnie made a red with his, Vafaei didn't. Does that make it 50% chance?
50% over the 2 occurrences we mentioned here, sure.
I'll show you some more occurrences, so we'll have some data to talk about:
This Quentin Hann you see here, was notorious for the 'smash break-off' in the nineties. (also notorious for match fixing, but that's another matter)
In this video he makes a red one time out of four break-offs, and to me he seems particularly happy it happens that one time.
Btw, also Jimmy White does a smash breakoff (and makes a red) but that was in a demonstration game.
Pay extra attention to the spread of the reds after the break-off, they are all over the place which makes the table a minefield if you want to put a nice break together (let alone a 147).
I'll admit, I said 'slim to none' to emphasize my point a little bit. The is a chance a red will drop, but the chance you give your opponent an open table is far larger so it's just generally never a good idea.
Come to think of it: I remember watching a 1P game where Corey Deuel broke the rack as an 8ball break where he hit the 2nd ball. I seem to remember he was about 50-50 successful.