The biggest disadvantage in my mind is that you're setting yourself up for a potentially huge merge just before the GHC release and might block the GHC release until that merge is done (assuming that haddock is still shipped with GHC).

 

Excellent point.

 

The merge shouldn’t block the release, though. In extremis, I guess we could always release the GHC fork of Haddock if the tip of Haddock wasn’t merged to match GHC!  But I doubt it’ll come to that