Seems to me that there are two distinct issues here:[list=1][*]Whether it's possible to reach a safe standard in 3 weeks[*]The standard of training in the US[/list=1]On the first point, I think it's important to realise that this is a very tight timescale, and will be hard work. I did my CPL in 3 weeks - that's a little over half the number of hours of the PPL, and I already had a reasonable amount of experience beforehand. Although I wouldn't say I struggled with the timescale, I wouldn't have been able to fly very many more hours in that timescale without suffering from an information overload and not taking on board everything I could have done. Whether you will be able to pass comfortably in this time will depend very much on your own abilities to absorb information under pressure. Plenty of people manage it, so it is obviously quite possible.
On the second point, the answer, basically, is that it depends where you go. There are some very good schools in the US (some of the best instruction I've ever receieved was in the US) and some very bad ones. Likewise, there are some very good schools in the UK, and some very bad ones.
However, even if you go to the best school in the US, and you are the fastest learner they've ever taught, it will still take a little while to get used to the differences in the UK. The main things which will take a little time are slightly different radio procedures, a different way of organising airspace, the differences in the culture of flying such as having to get prior permission before visiting another airfield and paying landing fees when you arrive (all minor things, but unnerving for someone with low hours). And the biggie - the weather in the UK is absolutely nothing like the weather in Florida or California.
I think that 10 hours, as some people have suggested, is a very long time to get used to flying in the UK unless the instruction given was particularly bad, but a couple of hours should be expected. I know that I needed a couple of hours to get used to the US way of doing things when I went over to America to do some flying shortly after I got my license in the UK.
One other point which someone mentioned was that the examiners at these schools often work for the school in question, and they therefore have a vested interest in passing people to make the school's statistics look better. I've experienced this first-hand where a school (coincidentally in the US, but not a JAR school) pretty much guaranteed me a rating for a set price. If I'd failed the test, they'd have had to offer me more training and a re-test - and since it was a fixed price, this would have cost them money. Come the day of the test, I was not really ready, and messed up one thing in particular. The examiner gave me another go, and with a bit of help from him I managed to get it almost right second time round, and he passed me. I'm very pleased that I didn't intend to actually use the rating (it was a sea-plane rating) - if I'd been planning on flying sea-planes I'd have certainly required more training before considering myself safe. I can't say that JAR schools do or do not operate in a similar way, and I'm sure that no examiner would ever pass someone who he considers completely unsafe.
FFF
---------------