We migrated a medium-to-large (in terms of model complexity, not rows), from PG to CR about a year ago. It went pretty smoothly, but you'll definetly run into issues. (Very happy overall, btw)
Every release has improved the compatibility story at an impressive rate.
The 20.2 released added partial indexes and enums, which helped close the gap in our app(though, enums don't support binary encoding yet, so might not work with your pg driver (1))
Things that are still an issue for us (all can be worked around):
1 - Can't defer foreign key checks
2 - No pg_trgm
3 - Can't tell if an upsert was an insert or an update
4 - No triggers (this is pretty huge)