Roblox data stores are prone to having many issues still and are not too reliable. I would recommend you look into caching to prevent data loss. Caching data allows you to only need to save the data when the player leaves reducing the likeness of ever hitting the limits. In addition, if it fails, then you still have all the data on the server and can continue to try to save the data until it is successful.
Essentially, you will grab the player’s data when they join and then store it on the server. Then simply update the data on the server for every change. When the player leaves, you will then keep attempting to save the data every few seconds until it is successful and then remove the cached data from the server. You can also autosave everyone’s data every 30-60 seconds in case Roblox servers ever go down.
@Kampfkarren has an awesome, open-source module for datastore caching you should look into!