@regehr @steve Anecdotally, there's at least 3 (Intel, AMD, Centaur) companies that do this on the regular, and one of them (Centaur) is quite small as such things go.
I wouldn't want to do it either, but the other thing you gotta keep in mind is that the CPU core, while important, is only part of a SoC and ISA has very little impact on the "everything else".