My current experience is that it unfortunately doesn't run on my PI 400. It doesn't boot. For a very short moment it's only a short text visible in the upper left corner ("don't power off" ... or something similar).
On my PI 4 (4 GB) it's running very well (for that a very big applause for the dev-team!!!)
Even if I try to start the PI 400 with the already "initialized" Recalbox SD-card that is running fine on the PI 4 the PI 400 doesn't boot.