This is one of those posts that I didn't originally consider writing, but given that the situation hasn't changed in the slightest, and that people are still unaware of the general problem, I guess it's time for a blogpost.
You may or may not know todo.txt. It is a very simple task management script, basically a thin wrapper around a single textfile that stores all your tasks. On desktop it is very easy to edit that file by hand, on mobile devices, todo.txt offers various apps that are wrappers around this plaintext format. Since this is just a file you're editing, it seems natural to use Dropbox to synchronize your tasks between your devices. In fact Dropbox is what the offered mobile apps integrate with.
Unfortunately, that idea comes with severe drawbacks. Just take two devices offline and edit your task list on either of them. Then take the devices online again, and let Dropbox sync. Of course Dropbox does not know anything about the file format, so it doesn't dare to merge and creates a separate file (for example todo (hostname's conflicted copy 2017-01-26).txt) that contains the version of your tasklist from the other device instead. So whenever you edit anything on two devices while they're offline, you're left with merging two textfiles manually.
This wouldn't be so bad in situations where you are directly interacting with the filesystem anyway, such as when editing the file on desktop with a normal texteditor. But when you use the mobile apps, the entire point is that you don't have to edit textfiles with a on-screen keyboard. At least the Android application doesn't do anything about this either. It ignores all the files that Dropbox may have created.
The leakiest abstractions are the most unixoid ones
Aesthetically, this separation of data editing and synchronization and the fact that you have very direct control and knowledge of the underlying datastructure is very appealing to the average Unix-Fan, and if I had to guess, the feel of control one gains from this design made todo.txt a success.
My main gripe is with a certain belief it creates: The idea that arbitrary file synchronization programs are suitable for synchronizing arbitrary data. Sometimes your constraints force you into a situation where you have to use Dropbox to sync your data, but it appears that most users don't consider the tradeoffs that come with this solution.