According to Hardkernel folks, it's stable. Last time I want to make a switch (again) I made a few tests and sometimes I could not boot.
If you know what you are doing is easy. Everything is packed (kernel, dtb, u-boot, root, headers, firmware) and it was explained how to do it manually in case armbian-config way of switching kernels and repositories fails. You can try, but if you fail you are on your own to do some forum searching and fix the problem.
