I think that with a problem statement of "I want to control my data and how third parties use my data" Solid makes a good deal of sense. Personal information should be owned by you and you should be able to take it from service to service as you see fit. The two problems that I find with Solid, based on my own limited understanding are as follows:

(1) Services don't need to give back to Solid - Services, Facebook, your medical provider, whatever else you are using, does not have a clear incentive to provide their own data on you back to your Solid pod. It is far easier for them to keep it: it lets them do offline processing, and it keeps you more locked into their service. I'm not sure how one would solve this issue.

(2) Much like mobile apps with excessive permissions and the abuse of tracking elements - I don't see how Solid prevents the abuse of its service. If Solid catches on and Facebook has a permissions check saying "Let Facebook do 'SELECT * FROM .;' on your Solid data, how many people will click yes? Even if you request it each time, once the data is copied out, it is out there and can be packaged and resold, used to build advertising profiles, etc. You're back to the original problem of not being able to limit access to your data, but with extra steps. Where I think this could be solved is by Solid not providing the data directly, but by being a service which can answer queries. Queries could be items such as "Does user like cats? y/n/m". Or it could be something like "Here is an anonymized dataset being built out. Please add your input to it." Replies to queries could also have an amount of deliberately wrong or misleading answers given, depending on the service and endpoint to obfuscate your personal data on places that don't need it. While this can still be abused, it raises the bar for abuse.