The 5 GHz band is regulated differently around the world. The problem is Wi-Fi units manufactured for use in the EU and UK use a different 5 GHz channel. Essentially, this means they may be a little more susceptible to interference than the channels US units are configured for, which may or may not cause you some headaches.
It's worth bearing in mind that you wouldn't be able to extend the network with additional US Wi-Fi points if you wanted to, that will almost certainly cause compatibility problems.
