That is to say that yes, it does have a bunch of proprietary stuff on top, but it also:
1. Runs Linux
2. Uses Linux systems, like BTRFS and LVM, to pool data
3. Provides SSH access
4. Has tons of packages that you might want to use (Squid, Node, Plex, etc.) available
5. Supports Docker, among other things, so you can run what you like.
So yeah, they have a lot of proprietary offerings on top of a generic Linux storage solution, but if you change your mind you can take that generic Linux storage solution somewhere else without being locked in to anything Synology provides. The only caveat is if you choose to use their all-the-way proprietary systems, like their Google Docs equivalent or their cloud music/drive/etc. stuff, all of which is still stored on the same accessible drives. You might lose your "music library" or "photo library" or "video library", but your music and photo and video files will still be accessible.
> Synology
I am perfectly willing to sacrifice some performance in exchange for not being locked into a vendor-specific proprietary NAS/SAN hardware platform or closed source software solution that's proprietary to that one vendor.