Oh well, time to add my bit.
You probably know what I am going to say.
I found the training in the USA fine and went on to fly many hours in Europe and the UK. Almost all of my hours for issue of CPL (self improver) were completed either in the USA or in the UK, in a non radio PFA aircraft. The CAA let me complete a non approved IR course despite only having 400 hours TT (most in the US). I did only about 10 hours total for that (including the test). I passed the UK GFT at about the same time.
I went straight from flying light aircraft to large commercial jets and was selected on my ability. Funnily enough, I was the only one on my course (the others were CAP 509 x 2 and an instructor) who was commended during my line training for my excellent RT!!!
Not bad considering virtually the only RT I had done was in the US.
I have a different viewpoint I can throw in here. Whilst hour building in the US I flew with quite a few UK trained PPLs.
The complete lack of ability of basic circuit planning and flying was amazing! With one chap, he could not get the hang of flying a circuit by looking out the window and clung desperately to his instruments.
He hadn't heard of looking out the window.
Once he had tried the "no instrument" approach, he improved, as did his landings.
It seemed amongst those I flew with that there was a total lack of knowledge of the aircraft systems (something which is drummed into you in the FAA syllabus).
RT and pre-flight planning, filing flight plans, talking on the radio in busy terminal areas, flying at UNICOM fields, were all big problems for the UK guys.
So I reckon maybe it works both ways.
I trained in the USA and did most of my flying there. My RT is excellent, and I have had no problems flying in the UK or elsewhere (apart from having to constantly scratch around VFR at low levels).
UK trained pilots came totally unstuck when abroad.
I suppose if you just want to fly in the UK, then training in the UK is sufficient.
If you want to be able to fly anywhere in the world, then consider training in the USA.
I am sure there are good and bad instructors in the USA as there are in the UK.
If the CAA think a school is good enough, it will approve it. This has happened, therefore the CAA are happy that the school is up to standard.