I would say as you mentioned just the knowledge and learning, which is no worse than any new tooling.
Of that the one draw back I have is within the query syntax itself on consumption and getting to grips with the query/mutation fragmentation syntax a bit of a steep learning curve but easily surmounted. :)
Where rest does win is often with simple get calls for micro data where mapping the fields will take double the time to schematics forward. In these cases I have abstracted the calls into a simple get endpoint alongside my app build to save the bloat on the frontend.
Very little other than that I can think of as far as cons.
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