I seem to remember one of the points was the way GNOMEs plugin system was implemented plugins have no isolation so one bad plugin can cause all sorts of problems.

They were also limited in things they wanted to do with the default window tiling ui.

Sorry bit vague it was ages ago I listened to the talk - probably this one: https://youtu.be/ioswlaxdhSA?si=DlVQ-jDu5G3A3Iru