I would rather say, the kernel-userland interface is stable. I think it is one of the strongest rule regarding the Linux kernel: don't break userland.

In the kernel itself, however, drivers have to be constantly maintained, which is a PITA if your driver is not in the mainline, but very nice if it is, especially if there is someone to do the job of maintaining it for you.

Again, choices are made, you can't have everything. The ones that led to Linux are good, we know that with the power of hindsight, seeing how successful it has became, not because everything about it is positive (it can't).