* no lifecycle management of any kind - if you're using it for backups you can't set "don't delete versions for 3 months", so if anyone takes hold of your key, you backups are gone. I relied on minio's lifecycle management for that but it's feature missing in garage (and to be fair, most other) S3
* no automatic mirroring (if you want to have second copy in something other than garage or just don't want to have a cluster but rather have more independent nodes)
* ACLs for access are VERY limited - can't make a key access only sub-path, can't make a "master key" (AFAIK, couldn't find an option) that can access all the buckets so the previous point is also harder - I can't easily use rclone to mirror entire instance somewhere else unless I write scrip iterating over buckets and adding them bucket by bucket to key ACK
* Web hosting features are extremely limited so you won't be able to say set CORS headers for the bucket
* No ability to set keys - you can only generate on inside garage or import garage-formatted one - which means you can't just migrate storage itself, you have to re-generate every key. It also makes automating it harder, in case of minio you can pre-generate key and pass then fed it to clients and to the minio key command, here you have to do the dance of "generate with tool" -> "scrape and put into DB" -> put onto clients.
Overall I like the software a lot but if you have setup that uses those features, beware.