Tried a couple more systems:
Ran a couple of games and demos on both the Spectrum(+2B) and MSX(Panasonic FS-A1WX), no problems seen so far. I also tried the SNES and Saturn, and while both are a *little* on the blurry and noisy side I'm more than willing to live with it as they still look pretty damn great.
I am impressed by how well it's working with the MSX. With my XRGB-2+, it would take several hard reboots before I'd get a stable picture; here it seems stable all the time.
The most annoying thing I've seen so far is the excessive amount of time it takes for the Framemeister to resync whenever some systems change video modes, like when the Saturn goes from 480i to 240p and back again, or when some systems (like the NES) are power-cycled. I'm willing to live with that for now, I think - although I do hope it will be possible to cut the resync time down to something slightly less annoying someday.