I heard some second-hand accounts that Hetzner's object storage was pretty slow even in the same location. For email, that sounded not too ideal.
Also, I only have 5 mailboxes right now holding less than 15GB of data total... S3 is still cheaper than the minimum at Hetzner since I don't need anything close to a TB.
The nice thing about S3-compatible blob storage is that it is extremely easy to move the objects to another provider (or roll your own MinIO or similar setup) and point your server to it. I initially went with the File-System since it made incremental backups much easier, but ultimately, S3-compatible opened me up to more options and setups.