The 50/60hz difference will not be a problem for modern equipment. I did not expect that European outlets would be OK in a North American residence but that would have been popular for most Europeans.
The voltage difference 230-240V will probably be OK, The grounding/Neutral system will not make any problems, many European places do still use a 230V system without a Neutral. Y transformer with 230V between the legs does have approx 130V from each leg to ground.
If you look at a typical EU outlet the plug may be turned "upside down" so it has to be fully insulated as it was live on both wires. When you have no Neutral it has to be a 2 pole breaker, and usually with GFCI function.
But as I read the answers here, a combination of 120 and 240 with US standards would be OK to NEC.
Probably OK with EU outlets, but not sure about that so far.
