I had an FAA PPL, -273. I also did a JAA MEP rating and a bit of revision/hour building, but the RN had taught me to fly properly by then.
JABI
That is half the problem. Operations are made so simple in the US that for the operations required in Europe students who come back can't cope with the extra restrictions and requiremens of UK airspace.
The rest of the problem is that the teaching in the US is not so thorough or structured (I had few if any board briefs before PPL lessons. I was surprised when I was taught to give them for every new lesson - I had assumed that was just military) and simply uses different flying techniques, and there is a lot less theory taught. The students are flying and not understanding what they are doing. For example they are sometimes navigating following a process mechanically without really knowing what it is they are doing.
On average someone trained for a PPL in the USA would have to do 10 hours training to fly the club aircraft I can sign off, 10 hours becoming a minimum if trained at certain cheaper US schools.