The biggest downside to R2 is there's no public access which means you need an intermediate server to proxy user requests to assets in R2
User <-- (you pay) --> Some Server <-- (free) --> R2
Interesting to see a couple use case scenarios that give the win to each, so there doesn't seem to be a clear cut overall winner. Just depends on what you are looking to do.
It seems for application storage R2 is the winner. But R2 does not target archival storage as much, where S3 or Backblaze would be better choices.