With regard to those UBports apps that don’t compile as native Manjaro apps, my guess is that, among other issues, those apps require confinement that’s better supported through packaging formats such as Clicks, Snaps, and/or Flatpaks.
As an outsider (who does not speak for UBports), I do hope that UBports will commit to repackaging various apps as Snaps. Similarly, I hope that Manjaro ARM will commit to the continued support of snapd and other dependencies needed for running Snaps on Manjaro ARM.