Power line frequency hasn't been directly used for much of anything ever since mechanical timers disappeared from video recorders. The only difference should be that the mains transformer in the device will be slightly warmer/cooler than on the "correct" frequency, but TV sets don't have any conventional iron core mains transformers to begin with.
Whether the MX5500 can sync to the different line/field frequencies of the different TV system (525/60 in the Americas, 625/50 in Europe) is a completely different thing; I have no idea, but maybe it can. Colour system difference doesn't matter if you use the RGB input in the SCART connector. The MX might have a multi-system colour decoder capable of both PAL and NTSC 3.58, in which case using S-video and composite is possible as well.
I've understood that most STB boxes and similar devices in the states have component outputs instead of RGB, which might be a problem. You cannot connect that directly to a European SCART input.
-mika