in sleep mode, i never have any crash with emunand...so yes it's stable (for me at least)...but i prefer to turn off the 3ds and launch the exploit again, for more battery hours, the gateway launch exploit is really easy, you need to adapt to that...
The problem is not adapting, the problem is I do not own a Gateway and I'm certainly not spending another $60 on another flash cart. My friend has one I can borrow though, so if I can use it once and leave it in emunand permanently then everything works out.