No major issues in going from 2011 sp5 to 2012 sp3.
Edrawings - just uninstall all previous versions and reinstall 2012.
I have upgraded 3 different companies from either 2010 or 2011 to 2012 and so far the only issues that I have encountered were tied to ToolBox.
I have the habbit of completely having ALL the users checkin thier files, unistalling the EPDM application, defragging the HD, cleaning the registry files, defragging the HD a second time, install the new EPDM client and then "Get Latest" on the library & templates/formats folders.
Although this adds a little extra work for me I have NEVER had issues and the users are really happy with the better performance due to a cleaned up system.