I’m not OP and am a dev, but also prefer flat files. Here’s my reasoning:

• versioning - I use snapshots in my filesystem (BTRFS), which is more than enough, and have a git hosting solution for things I care about more
• sync is plenty fast on OCIS and Samba, it’s just kinda slow on Nextcloud; I’m sure Seafile is better, but it’s not something I do frequently anyway, especially since backups from devices is automatic and uses a different, fast system
• incremental - not my use case, most of my files either never change (movies) or are small (text flees)

My main concerns with Seafile specifically are:

• developed by a Chinese company and doesn’t seem particularly open to contributions
• mostly written in C, so there’s a good chance of security vulnerabilities
• documentation about the disk format isn’t very open, so third party tools don’t really exist
• main target is larger orgs, so I’m unlikely to get very good support

With flat files, I can easily switch to a different service if my needs change.