To be fair, outside of GPUs there really isn't much need for third party cards, and arguably even GPUs aren't a show stopper with third party GPU cages. But really for 99% of the use cases the Apple silicon GPUs are good enough. Nobody sane is buying a mac pro to run games, and for AI thats a whole different complicated set of reasonings (for training you'll always be better off with a datacenter server and abank $15K datacenter GPUs.). For everything else, the Apple silicon GPU seems to punch above its weight class.
Oh I suppose there is also video intake cards. I know my father was consulting on a job (he's an audio and video engineer who designs radio and TV studios) where they had mac pro and some black magic cards that had a whole boatload of hdmi signals coming in, and they've had to migrate to a PC. But I think increasingly there are viable thunderbolt solutions for that.