I hold both the EASA FI and FAA CFI.
Some things to know:
In order to have an FAA CFI certificate, you need to first have an FAA Commercial AND instrument rating. You will have to pass both knowledge tests, meet the requirements, and pass the flight tests.
In order to pass the CFI flight test, you first need the two knowledge exams completed (CFI Airplane and Fundamentals of Instructing), and there is no minimum requirements for the flight training, so your time sitting in the right hand seat during your EASA FI training will save you some time adapting to the right side. The tough part is doing the FAA commercial maneuvers if you have not done these before. The other tough part is the FAA oral exam. You can expect a mental beating to last about 5-8 hours or longer. The ATPL knowledge you gained in Europe will come a long way, but you need to get spooled up on the FAA way of explaining things, particularly the FARs (know these inside and out, mostly but not limited to Part 61 and part 91, also NTSB reporting requirements), knowing the AIM, and the FOI. You will also need to memorize both the Private Pilot and Commercial Pilot Practical Test Standards which can be found for free on the FAA website. You will lastly need to develop your own lesson plans.