Both of them have issues depending on the setup. X11 has worked flawlessly in my experience. Wayland has worked the same for most.
Personally, Wayland still has some growing pains, especially in regards to Input handling (mouse, keyboard, etc). In X11 it was "trivial" to edit one file and have the settings stick across different WMs (switching from DWM to i3, etc.) There's no standard for this with Wayland since it's up to the compositor to handle these things, meaning you're relearning how to do something as basic as setting pointer speed each time you try a new compositor. This is my only real fair gripe about it currently, as the rest of my complaints are just due to how young a lot of the Wayland-specific tools are - this will improve with time.
I use Firefly for business and personal finance purposes. I do my books nightly via a script that imports line items from the sales platform I use. Firefly's automation allows easy tag/category assignment based on any number of details (source/destination account, transaction description, etc.) A tag in my case is just "business name" (personal expenses have no tag), and categories are expense types. At the end of the tax period I can generate a report that I copy-paste into my tax software. So far all of my numbers have lined up perfectly across the board, but I also balance accounts by hand to make sure. Biggest problem is backing it all up and testing the backup. I backup the database nightly and test the latest backup every 2 weeks, at which point it goes to the cloud. I need to automate that.
The developer is also very active and there's regular releases.