At the 30 000 ft level I agree with what you posted already
Piko quality ok, but not LGB level, hard to compare to USAT
USA Trains - Overall quality ok, but not bulletproof like LGB.
Where this gets a bit tricky is in two areas - one is price vs quality and the other is quality over time.
I think Piko is at the 'sweet spot' when it comes to price vs quality - that is certainly reflected in their sales numbers both in Europe and the US. Piko also seems to be receptive to feedback from customers, taken their input and has improved the quality of their products over the years. For instance, early products had problems with the chrome plating on their locos and that got fixed.
LGB under EPL, ie year 2006 and earlier, for the most part was "bullet-proof" if the product was manufactured in Germany. People are still running the original Stainz introduced in 1968. LGB China manufactured products were still good, but not as good as the German ones - different plastics for instance.
Then when Marklin took over LGB in 2008 with manufacturing in Hungary, there were quite a few teething problems both in design, they introduced a German E10 Electric loco with the motor mounted vertically (like a lot of H0 locos) for instance which turned out to be a bit of a disaster and also in quality control where product was shipped out with parts missing - I remember a loco with one axle reversed in the motor block so the gears meshed with nothing or couplers on cars mounted upside down. But much of that has been fixed now and the LGB quality under Märklin has improved since 2008.
Don't have much experience about USA Trains quality - one good thing I think is that their American Series is manufactured in the US, I think their quality has been pretty consistent over the years. Well, that's to be expected - there is a fairly small number of different types of rolling stock - new items of the American series are just repaints.