I think that plumbing failures in a standalone spa would be relatively rare. Like every thing else, one failure does not make a trend and I suspect that the quality of different spas can be very different..
My spa has what appears to me to be schedule 40 fittings and plenty of primer drippings.
I suspect that plumbing failures on a standalone spa are no more common than what occurs on a pool or integrated spa.
Either way, I would not let the fear of having a broken pipe stop me from buying the kind of spa I wanted, either standalone or integrated..
Thanks,
Jim R.