The same could happen with Dropbox/icloud, or even if you just ran two Minecraft instances with the same copy of the world on one machine - Minecraft worlds are not designed for concurrent modification by multiple processes

The same can't happen with Dropbox, because minecraft worlds are NOT stored in ~/Dropbox/ folder. That actually reinforces my point about a separate synced folder being a plus for most users, not a minus.

> minecraft worlds are NOT stored in ~/Dropbox/ folder.

They certainly can be. They can be stored wherever you like, either if you are using a modding/hacking launcher (both are very popular for non-technical players, e.g. kids who aren't programmers) or if you are running a minecraft server (the vanilla server jar runs wherever you decide to put it, and doing this is not exactly an esoteric feat; again, nontechnical kids do it.)

Of course they can. However, person who knows simlink-fu and other tricks is unlikely to find himself in a situation when his data is corrupted by an incorrect Dropbox sync process. Syncthing's user, however, can screw himself with just a few mouse clicks, and I personally saw that happen.

So I myself simply sync just one folder with Syncthing, Dropbox style.

I don't think there's any meaningful difference in difficulty in finding the Minecraft folder in AppData to sync with Syncthing vs finding the option in the minecraft launcher to save to your Dropbox folder.

Third party launchers commonly used for modpacks, the first party dedicated server, changing the settings on the first party launcher are all ways you can put your saves in the Dropbox folder

So the caveat is that concurrently writing to the same file on separate machines will lead to data corruption? Is that reproducible with other files than Minecraft saves, or can it be that Minecraft introduces corruption by amending the on-disk file with a delta of currently-running changes not accounting for the possibility that the file has been altered?

It's a common problem with git as well. Running git in a dropbox folder is discouraged.

Wouldn’t that happen with Dropbox too? Maybe the race condition just happens less often because it syncs slower?

No, because you'll have to do quite a few steps to put an actively used minecraft world in a ~/Dropbox/ folder.

That doesn't argue that Dropbox won't also ruin actively updated folders. You're now talking about Minecraft configuration which is orthogonal to your original point.