Primarily, they have very low resources (and/or skills, it's unclear), which is a problem, since resources need to be distributed between bug fixes, improvements, and chasing library updates (see: GTK3 updates).

A few days ago I've booted the 19.04 beta Xubuntu live image, and it was showing minuscule fonts on a 3000x2000 screen. This is a bit embarrassing for a stable DE.

I don't discourage it in an active way, but one should consider it very carefully (there are alternatives nowadays, that are equally lean, but more stable and flexibile).