The downside of emulating a CPU in ROM is mainly that ROMs aren’t all that fast. Though, if it is all one has, it is better then nothing. In terms of the Gigatron, there is yet one more downside.

I respectfully disagree. In the 40 years or so that I’ve been using computers, a “microcomputer” was defined by its physical size, about small enough to fit on a desk or smaller.