In my limited experience with Hilton, I can recognise the above view: the few Hilton properties I've stayed in in the USA were in a bad state and off, while most Hilton properties in Europe are quite luxurious. I would tend to think its a clear choice from the brand, serving different markets with different needs.
There are of course exceptions everywhere: The Hilton Avisford Park in the UK was a threadbare dump with mediocre service (IMO) that I was a touch embarrassed to have brought my wife to, while properties in London, Tokyo, Sydney, etc. have exceeded my expectations. Vancouver (Richmond), Toronto, Sao Paulo and Rio met but didn't exceed. Conversely in the US I tend not to stay at mainline Hiltons with as much frequency but the Times Square was always an above average stay until the past few years the property was open which saw a noticable decline the Hilton Burlington was a disappointment, the rest are solidly in the middle of the pack meeting expectations which are, I will admit, lower for US properties -- part of the reason I don't frequently see the advantage of Hilton over a more conveniently locate HIS/HGI in much of the CONUS