Here is my take on all this.
I've concerted a few US trained guys back to CAA licences, and they turned out to be pretty good. However, the sample size was small, and they were low end CPL / Instructors at approx 1500 hrs, rather than raw PPLs.
I've also done a bit of flying in the states, hiring aircraft from 2 flying schools at which europeans were doing PPLs. And I have to say that I was concerned by the quality of the courses they were receiving. There were some perfectly capable instructors, but they were working flat out and the overall supervision of the course just wasn't there. In fact, both schools had recently qualified PPLs write of an aircraft, in one case fatally. One of these schools is now out of business, and the other is somewhat notorious on this site. Mentioning no names but its on the east coast of Florida....
Now, I accept that my experience of the US situation has been pretty negative, especially because of the fatality, and 2 schools is not a representative sample. Nevertheless, I'd be lying if I said that the overall quality didn't concern me. Even so, that does not mean all US trained pilots are crap any more than it means all UK trained pilots are perfect!
Bleh. All I've done so far is repeat things others have already said. Time for something different...
I reckon students who go across to the UK to train for a PPL in a concentrated lump have got 2 big things going against them:
1. Stuff is different.
2. Knowledge Retention (or lack thereof).
Stuff Is Different.
My pet theory is that the difference between a flight that is going fine, and another flight that goes to worms, can often be a minor increase in workload. In the first case, you've got a bit of spare capacity, in the other you're 'maxed out'.
So when a student suddenly seems to 'click', and starts to be able to hack it, it looks like they've somehow radically improved their skill. But I reckon that what is actually happening is the student is gradually learning stuff, and eventually makes the transition between working at 101% capacity and working at 99% capacity.
As you get more experienced, you have more spare capacity. But the newly qualified PPL holder doesn't, and therefore anything which requires brain power may well nudge the poor sod back over the 100% workload barrier and bring everything tumbling down.
Now your US trained PPL getting checked out in the UK (*or vice versa*) has an accumulation of stuff which is different. OK, a Cessna - 152 is still the same aircraft, but we all know that aircraft handling is only a small part, (albeit an important one) of being a pilot.
The RT is different. The charts are different. The sorts of nav features which are good fixes are different. Instrument / Comm / Nav fit may be different. Sources of weather data are different. None of these changes is staggering, but its 'death of a thousand cuts' territory.
A more experienced pilot has the spare capacity to take these changes in his/her stride. But the newly qualified PPL, by and large, does not. Therefore they are likely to fly like a chimp.
In other words, I reckon a would be PPL would be wise to train in the environment they plan to fly in after they have completed their training. Then build up experience in the same environment. Then branch out.
Knowledge Retention (or lack thereof)
I'm not a big fan of people doing PPLs in concentrated courses.
Its a bit like cramming for an exam. The student may well have enough knowledge / skills stuffed inside his head to complete the course, but how well will it be retained?
Especially if the savings have all just been spent in one lump, and it will be a while before things can be consolidated once the new PPL returns to their country of origin.
I reckon its better to accept the uncertainties of the UK weather. If the student can't fly, theres no end of other things they can productively do. Groundschool, Briefings, Practice drills, etc.
Spread the course out, there more to it than the time spent in the air.
So basically, I'm saying that even assuming quality of instruction is identical in the US and UK, I reckon US trained pilots will perform poorly when first flying in the UK. Obviously it is a sweeping generalisation, but I think there are inherent factors which make it likely.
Oh, and as I mentioned above, the same factors would apply the other way around, but I don't suppose many Americans decide to visit the UK to do a PPL!
In completion, the first time I visited the States I had about 200 hrs of experience. The combination of small changes in the environment, compared to what I was used to, did make me feel like I was flying slightly below par. It was a feeling which rapidly went away, but I'm sure that the check flight I did on arrival was not my most polished performance.
CPB