Oh, I have plenty of reasons against Flatpak :)
Unnecessary duplication of system dependencies, audio tools not able to use system paths for opening files (like package manager installed plugins) to unreasonable devs that don’t take input from upstream developers, maintainers or users.
Call me a boomer, but I prefer the “traditional” way of building, installing and running applications and packages :)
Btw afaik a flatpak won’t help with the glibc issue.
I won't call you anything. Everyone chooses their own poison. 😉
For me, as an app developer, Flatpak is an extremely convenient distribution target, since it lets me ship the same environment to the user that was tested working. So the dependency problems you described (including glibc) are avoided on any system.
For your case, my best advice would be to build and install the development libs for a newer version of SDL2, and then build and install midiconn from source. The other dependencies should be at sufficient versions on Ubuntu 20.04 as well.
