"Proprietary parts would be major components such as printed circuit boards (PCB’s), compressors, or fans (indoor or outdoor). This goes for the majority of manufacturers. Capacitors, contactors, pressure switches, etc. can be replaced with aftermarket parts."
This is why you avoid inverters. That, and their high cost no matter who makes it. The benefit they offer is near-meaningless. What they are is the product of a group of engineers who needs jobs, and marketing execs who need to sell something new and shiny. Avoid it all like the plague.
As for traditional units....about 20 years ago our unit failed and I had an AC guy come out. He changed the run cap and the unit worked as intended. I paid him his service fee...plus his bill for the $125 cap. After he left...I started to ponder this and decided to look up on Grainger (probably the highest cost source of all) what a new name brand cap would cost. $8. I called the AC guy and he gave me the usual line about how the need to make their money....I told him I didn't expect him to charge me $8 but 1200% markup was a bit dear. That is the last time I ever hired an AC guy to do anything. The money I've saved has, I'm sure, funded at least one new pinball machine.