Judging by past experience, culture gap is more of a problem.
Could you expand a little? Are you referring to the workaholism some Americans have? Personally, I've always sought out companies that value a healthy work/life balance and am relieved to hear Europeans companies value it so highly.
Aside from being too enthusiastic about work, what other gaps in engineering/work culture exist?
I used to work for a company that has both US and EU offices. The EU ones have considerably better WLB (but of course less pay) and people take a t least a month of vacation during the summer. Aside from the workaholism issue you talked about, there is a more subtle difference that I felt -- US has a stronger marketing culture while the EU has a stronger engineering culture. However, this is just an isolated case.