the blog post inside the linked blog post goes over some points. each point is copy and pasted more or less.

• like for example the multiple times I’ve spent dozens of hours debugging a single issue only for it to turn out a small typo or a careless mistake that any language would catch at compile time, except for C
• Memory safety issues arising from the absolute lack of any documentation whatsoever of wlroots have also been quite the annoyance
• The development of a display server is very complicated, as they are very broad and complex pieces of software. Mixing a C library with 0 documentation is basically asking for trouble.
• new wayland features that require changes in wlroots tend to take ages to get merged into wlroots, like for example tearing, where a basically ready MR took 9 months to merge
• explicit sync still not being a thing, despite KDE and Gnome having implementations already (I believe it is now, but not at the time of the blog post)