Okay, this is the third time I've tried to answer Paddler's question.
Maybe I should have said that some manufacturers have had a problem, or two with the plastic they use. In particular, Eddyline had a problem with the plastic gaining humidity while in storage. This caused some kind of problem that would show up after a time in the finished boat. I read where they replaced some of those boats.
As I said, I have no problem with thermoformed platic as a boat building material. It just might be the toughest and most durable material presently being used. But, like everything else, there are pluses and minuses. From some of the designs I've seen, it appears that it is necessary to design in some divits and curves to keep things ridged. Nothing wrong with that except that some places on hulls are relatively flat and these areas tend to be a bit flexible. That of course is not unique to thermoplatic, but I have noticed that Delta for instance had to put a stiffener in their hull.
If there is one issue with thermoplatic that looks to be a little bit of a problem, it might be in forming in a very sharp bend. I'm sure somebody has figured out how to do it, but so far I haven't seen it.