I don't think radio use is an issue.
I did my IR in Arizona.
Yes it is true that America is less formal than the UK or Europe, but to be honest the standard of PPL VFR radio usage in the UK is so often so utterly appalling (and I am not talking about new pilots) that somebody who learnt the radio in Outer Mongolia is going to absolutely shine upon coming back to the UK.
The upsides of doing stuff in the USA have already been well covered. The principal downside for some people is that they will have trained in a different type of plane to what they will be flying over here (either as renters or as owners) and that is worth a good number of hours in itself.
This factor is the main reason why so many UK/European owners have been trying so hard, spending so much money, and so often unsuccessfully especially in recent years, to arrange for a European FAA checkride having trained here in their own plane. Most gave up and went to the USA in the end.
For somebody doing an ab initio PPL this is not really an issue, so long as they stick to some common type like a LA28 or C172.