For instance, say I give Solid Social access to my contacts and my photos. At that point, Solid Social can access all my contacts and photos, copy them, and then do whatever they please with them (even if I later revoke permissions). In the current web, I upload my photos to a website, which I then trust not to use them inappropriately. In the Solid world, it seems like I give photo-access to a website (which could then copy them) and trust that website to not use them inappropriately. This seems like the current system, but with extra steps.
In terms of data compatibility, I think that there's a potentially compelling case (user-driven standards vs. bespoke handling per website), but again I fail to see the practical implications. For example, if I upload a video to a website and it becomes incompatible with other websites, my assumption is that there's some reason for that step. It could be out of walled-garden malice, but it also could be that the website is encoding it and co-locating it in such a way that it'll be really fast for other users to watch. In Solid-world, it's unclear what the flow here should be: does the website only get to consume the videos as I have them on my pod (potentially terrible performance)? Does the website get to still do optimizations (and potentially keep artifacts of my video after I revoke permissions?)
I like the drive towards consistent standards for interfacing with data, but other than that I could just be missing the important bits here (and please let me know if so), and I fail to see the positive change in most web-based user-experiences for the most part.