My personal laptop and desktop are on btrfs and my work laptop and desktop are still on extra; I haven't noticed any difference between how well it syncs between those sets of machines. In fact, after the initial setup of it on a Digital Ocean VPS (20 GB is enough for my uses), it's been virtually the exact same experience as Dropbox for me; the initial sync on each my my machines too roughly the same amount of time as the Dropbox sync did, and every change since then has been essentially instantaneous to my other devices. As an Arch user, it's actually easier to install than Dropbox (nextcloud-client is in the main repos, whereas Dropbox is only in the AUR), and the Android app has also worked perfectly for me. Assuming you're okay with a bit of upfront cost to get the server set up (I basically just followed the Arch wikis and then used Let's Encrypt for the TLS certificate) and the cost of a VPS, I'd highly recommend it for people who want filesystem sync'ing for non-ext4 on Linux.