True. I suppose the problem with BIOS updates is mostly the diversity of the hardware the BIOS is used with, and the near-impossibility of thoroughly testing it, not with all conditions found during use, but with all conditions found *at boot* or even during the running of an update: hence the relative likelihood of boot-time failures bricking your system after a BIOS upgrade.
I do wonder, though, how much of the set-top boxes' firmware is actually upgraded by an update. If the components necessary to boot and do an update don't change, then that might reduce the likelihood of failure -- though the fact that we rarely see them fail in any way suggests that this is not the problem.
So... continuous upgrading works for stuff (even complex stuff) that runs in simple or consistent environments or that is not itself necessary to run the upgrade process. That's probably enough for embedded stuff, since their environments are normally nailed down by the vendor and all variations known. Just look out for embedded systems that run as part of larger systems, and whose upgrading is controlled by the upstream vendor: those may not have been tested in the environment they're being upgraded in. (This, too, is probably rare: the integrator would probably want to control the upgrade stream in such a case, since it's them on the line if things go wrong.)