Ask HN: Can Someone Explain Syncing via iCloud?

There is a to-do list app in the apple ecosystem called DoneIt. I really want to love this app, but it's plagued by several things.

One of them being Sync.

It's advertised as being private because it uses your own iCloud Drive as the database.

However, it has atrocious syncing:

1. Changes or additions get lost 2. Very slow for things to reflect 3. Just doesn't work

I know iCloud isnt ideal. But other apps like 2Do use dropbox (can also use iCloud) and work just fine.

How would you fix this? What are the issues with making this work? How would you implement it?

I am not a software dev by any means but I have general knowledge. I know there are things like merge conflicts, race conditions, etc to keep in mind.