I see no disadvantage of doing the FI(A) in the states. From memory only IFT and EFT in the states have the approval.
There is currently an FI(E) in the states (Mr Gill), so that completes the picture.
I did mine about 3 years ago in the states and have always been employed, now with a large UK integrated FTO as a CPL / IR instructor.
If you do goto the states, go with your eyes open. After all the travelling and additional expenses, there really is nothing in it between the UK and the USA for the FI(A) rating.
I really enjoyed the course and once you get it, hopefully you'll find some work and you won't need to pay for your hours anymore.
Good luck.