To be probably more generic, I could probably formulate it the other way: if you have one window crossing multiple desktops (for instance you just move one window a little and it starts to overlap between two virtual desktops), then its bbehaviour starts to become erratic concerning its position.
If you can connect from another computer, then you see that Xorg is using 100% CPU, it is not able to handle this vt switch. This is not the whole system per se, but this behaviour is really a pain.
A as sidenote, I'm not really a developper but a researcher which uses this computer daily for more than 15 years for doing research. I have to use Ubuntu since I'm not the administrator of the machine, and I have limited power on my system, so I keep going with Ubuntu 12.04.4 LTS ...
I've used many distributions in the past, including Fedora, Mandriva, Mint, Debian, openSUSE, ... and I have never encountered such iodiosyncratic behaviours using the WM they provided, even in GNOME 1 and before, or KDE 1 and after.
I'm using on a regularly basis 3D rendering for viewing molecules, and there the system crashes (I mean Xorg + proprietary drivers). Not everything is related to Ubuntu + Unity itself, but I see those crashes happening more regularly using Unity.
The issues observed in third-party applications (libreoffice for instance) were not observed in the past using other distributions.
This is not FUD, only facts I am encountering. If time permits, I will submit a detailed article on this to Michael, since for a more advanced usage than "developping", some aspects are really weird using Ubuntu.
Last, I have written more than 50 articles on distributions in a paper magazine, so concerning performances and drawbacks, I can put figure on words (search for me at http://www.linuxidentity.com/ ).