The problem is it only takes one bad or incomplete API needed for your specific use case. I ran into this a lot when I used cpal. For example, the data stream enum type (i16, u8, f32, etc) didn't have Hash or Eq derived, so I had to create a wrapper class for the data stream type. But, the type was marked non exhaustive, so I wouldn't be able to tell if my wrapper would get out of sync with theirs. It was a pain to work around.

In other cases, I couldn't work around, so I had to vendor some things. I ended up implementing my own graph library, because the existing one wouldn't let me reach into change some invariants for undo/redo. Which I mean, fair enough if that's what's needed, but it's a real pain to reimplement something because the API was incomplete. And of course, now if something from another library needs petgraph, I'd have to convert my graph to its graph.

So yes, in theory, if we had great APIs this wouldn't be a problem. Unfortunately, APIs are always a work in progress, and sometimes need escape hatches in order to send values between libraries.