To flat shift correctly you'll need a lot of skill (more than I have, and I'm pretty adept at heel / toe etc), and a very light flywheel, or else the rpm will be too high when you try to select the higher gear, and the synchros will resist the shift, and / or you'll crunch the gears.

All in all it's a great way to wreck your gearbox, they just aren't designed to do it. Racing 'crash' boxes are different...