It is quite stable and runs on Python, which is a very stable base.
The developers are very familiar with Linux and I am sure they would not go with Flatpak without good reason.
Flatpak is generally about packaging all needed extra libraries/dependencies into a package to prevent issues with dependencies missing, wrong version, etc, to enable apps to work across any Linux "distribution" (distros) and also to enhance security by providing some sort of "sandboxing".
As far as I can see, however, there are also instructions to install versions for various different Linux distros and I believe this Flatpak being the last option is to cater for various other platforms like OpenSuse, Alpine etc.
https://openlp.org/#downloads
But you can also just google something like "OpenLP Suse" and for various distros you will find packages specific to that distro, such as https://software.opensuse.org/package/OpenLP or https://packagehub.suse.com/packages/OpenLP/ - but the main problem with these distro specific releases is that many of them are community maintained and will lag behind the latest version.
I discussed some other possible options for Linux that involve a more manual install process in this recent post, and this might help with keeping up to date but comes at the cost of manual intervention and need for a little know-how.
https://discuss.openlp.org/d/6814-linux-mint-213-pulling-the-old-version-of-openlp/2
Please note though I am only a user like you, not a developer or expert, so while I think my information is correct, you may have to experiment a little.
