The SFC has a 9V psu, and shares the same plug as a Genesis/Megadrive 2, if i recall. The US SNES uses a proprietary plug (the fear was that people would plug in the adapter from the NES, which outputs AC power instead of DC power) and states 10V.
The US NES (not sure about pal, but I assume it's the same) has a rectifier inside, so it's pretty lenient on what it can take. You can plug in an AC PSU, it it'll convert to DC. Likewise, you can pump in a DC psu as well (so long as it's 9-12V... 9V preferred), and it'll take that fine as well. Hell, you don't even need to worry about polarity for the US NES. I seem to recall that the PAL SNES has the same setup with a rectifier inside.
But, if you plug in an AC PSU into a console that doesn't have a rectifier, then that's bad news.