Isn't rebooting during a flash pretty much the worst possible idea?
I've done this a million times on server hardware. Never with an ASIC. The downtime in case of an issue and probable lack of being able to return it is what scares me.
All you are flashing is the router really =) nothing else. It should be fine, I doubt it has a very high failure rate... I havent heard of any known flashing failures on avalon, under normal conditions.
But yeah, its just like flashing a mobo, same chances of failing I suppose... which over my PC career seems to be 1% of flashes I do, something breaks =P