Fossify had quite a bumby ride. It is a fork of a oneman show that was called Simple Mobile Tools, whom was sold off, the license didnt seem to stop them doing that see Simple Mobile Tools to be bought by ZipoApps
You should check the issues on github for their various apps to see how massively unstable these apps are. I used to use the apps based on other recommendations and the often caused issues in losing calendar appointments and contacts, as well as inaccuratly displaying data. I have given up on the project as it seems to be build in a way that does not deem work reliably.
Generally spreaking. You need backups, but especially when you keep everything offline. Make sure to test that the backup file actually contains all the data… (Yes they had issues with this too)