Maybe that kind of situation is fine and the question is really just if there are queries that biff.graph can handle which Pathom can't. If your resolvers are written correctly maybe not? But there have definitely been times with Pathom where I did something wrong that threw off the query planner in ways I didn't expect.
In any case, if I end up wanting to support migrating easily between the two as a core feature, I'd definitely want to e.g. do a bunch of generative tests to find out what kinds of queries end up with different results. Until then, a downside of supporting Pathom resolvers without a shim is that it might give people the false impression that biff.graph is a drop-in replacement for Pathom or vice-versa.
So far though the main target audience I have for biff.graph is people (biff users) who have never even heard of Pathom before, so interchangeability hasn't been a top concern. Though if many people start using biff 2 and then eventually some of the start wanting to migrate to pathom, I'd be down to explore that area.
And haha yeah nice to bump into you again--I think I remembered your username from reddit, assuming it's geokon there.