I think there are many good tub makers who use generic parts from various suppliers. Some of them work fine. However, the availability of those parts in the long run is not guaranteed any more or less than specialty parts. If the vendor decides to change the design, or simply goes out of business - or much more likely, gets bought out by another vendor - the generic parts you may need will disappear.
I have worked recently on a HotSpring tub which was around 25 years old. They had jets made by HydroAir. They were off-the-shelf units, but they are no longer available. The good news was that the factory had a procedure for fixing the problem without replacing the jet - I obtained a special adhesive kit which came complete with photographic directions, stirring stick and even a couple of gloves to keep the stuff off my hands. I did as directed - I've been married for 27 years now so I know how to follow directions - and closed up the cracked jet fitting from the front with the materials provided. The tub is back in service.
Older models with heater problems have elements still available dating back to the eighties, housings are in stock for most models - or upgrades are available which allow replacing the heater element and housing with a complete new heater which resets on power down for even greater safety and ease of use. Items which proved to give less than satisfactory performance have been replace with upgrades or direct replacement parts.
I would say that if you stick with a brand-name tub, parts problems are not going to be an issue now or in the future.
8-)
