I'd like to sync my markdown notes between devices (laptop and phone), which service is better: Nextcloud or Syncthing? Any other important idea I should know?, like latency times, or maximum number of synced devices, what if I edit the same note from both places without internet and then both get connected to a network... For example, I know Nextcloud let me have a history of the notes.
First, the rough stuff. Syncthing is not user friendly to set up and use. If you want to sync something, you have to go to every device you want to share with and enable the share... for every folder. You can get through this by doing the Dropbox method of just sharing a single folder, but that will have it's own challenges if you want to only share certain files with certain devices or apply different versioning methodologies. All in all, it can get cumbersome.
Secondly, it's a dumb sync tool, meaning that it's not content aware. In your "editing the same note" example, Syncthing will fail. You'll get a conflict notification and be prompted to to pick which version is correct... and even that will be buried in Syncthing's interface that you'll have to go looking for. That specific scenario isn't what it was designed for.
Lastly, Syncthing will occasionally get hung up on syncing a file and clearing the error isn't straight forward.
Alright now for the good stuff. Once you get through all the above stuff, it just works. I throw it on every computer, phone and tablet I own, and I get (mostly) worry-free backups of all my important files synced to my NAS. And it has saved me MANY times.
There are multiple versioning methods, so you can be sure that if you accidentally delete something, it'll be there for you. You can set it to encrypt files on specific destinations, so if you wanted to sync to a VPC, you can ensure your folders are protected from prying eyes. You can exclude files, so those annoying .DS_Store files that macOS throws around don't end up on your other devices.
I use it to backup my Obsidian vault, but I don't use it to sync my vault to other computers. I pay for Obsidian Sync for that.
best description I have ever read of syncthing. they should put this in their readme or about page. describes the pros & cons in an honest way. I've had all the problems you describe and will probably have them again in the future, be confounded, be frustrated.
I get (mostly) worry-free backups
except this; it's sync not back up. ;) but it is very backup-like and in some situations does the job as well or better than backups
I mostly agree with @shadejinx. I would add that when editing the same note on two separate devices syncthing will at least fail kind of gracefully in that you'll whatever.md will still be there but you'll see an additional whatever-conflict-hash.md along side it so you can easily fix it up. Synctrayzor for windows will give you a nice notification and UI with which to resolve.
Nextcloud is great but it's a real behemoth. Loads of stuff you don't need.
IDK what you mean exactly but I sync between computers and devices just fine.
I sync multiple computers but not directly. Everything goes to the NAS and then anything I want to share comes from the NAS. That way versioning is on one device and not spread out all over.