Risk of non-driver bugs

I would mostly worry about filesystems and memory management. The experimental kernel is going to unpack and copy all of essential OS files. Filesystem bugs are not so rare even in late release candidates.

Letting the users choose would probably the best approach in terms of user experience, but it would require a lot of work. Should the user select the kernel in the bootloader or in the installer? The bootloader is hard to make polished and user friendly. In the latter case, the chosen kernel will need to be loader with kexec. Then how do users stay with their chosen kernel variety as they run updates? And how do they switch from one variety to another if they want to?