Consider that the NEWEST one is 25 years old and they can be up to 40 with, I suspect, the majority being well over 30 years old. Given that fact, how badly could they have been built?
How many other products from the early to mid 70's do you own or even see around? Houses and collector cars are about it (I do
still have my Marantz & Altec Lansing stereo
Old GRP boats are incredibly long lived. A large number of tall tales and old wives tales about the "quality" or lack thereof of the build from back in the day have survived until now. AFAIAC they have been proven meaningless by the continued survival of these GOB's.
Check out the one you are interested in like any other boat you might consider and forget about the stories - most of THEM
are 30 to 40 years old as well.