The fact that it is difficult to personalize, firstly. Coming from Plasma and before that Zorin, I find it hard to change the look and feel of the OS.

Then there is the package manager. TBH, after having the Pacman/AUR combo, nothing ever comes close to it in terms of simplicity.

Plus I don't know but I find it bloated and laggy. But then again, maybe it's just me. That's the beautiful thing about GNU/Linux, it's that yog can choose the distro that fits your needs.