That's sort of a chicken-and-egg problem. If residential three-phase electricity were commonplace, electricians would all happily handle it at their standard rates, your air conditioners and refrigerators would all be running on it.

No? It clearly takes more training and has more liability since the consequences of a mistake are more serious. The average billable rate would inevitably be higher regardless.

What kinds of mistakes are harmless at 120V @ 100A vs 400V @ 30A?

Touching 120V with dry skin is nearly always harmless. 240V is considerably more exciting.

Maybe it's a little different for just a converter, but mine didn't have a problem installing a Phase Perfect and running a few three-phase outlets for it.

Wait, there are special electricians for three-phase in the US, is this like an additional certification or are the normal ones scared of three phases?

Not sure if there's an additional certification, but it would not surprise me at all that an electrician here would not want to work on a home with an unusual setup that they're not particularly familiar with.

Electricians specialize. Resi guys do residential and rarely to never handle 3 phase. I do commercial and light industrial because heavy industry isn't around here and I despise doing resi work. Three phase is usually run everywhere by the power co and a transformer (pole pig) converts to single phase near the residence.

There isn't. But residential electricians are specialists. Then there are commercial and industrial ones, etc, etc.