Regarding working with 'all' extensions. It's not that easy. Basically every extension developer has developed their extensions so their admin looks right with khepri. Anything different from khepri in any way, is going to potentially cause some rendering issues. The further you move away from khepri the more problems you are going to potentially have. EVERY 3rd party admin template is going to face this same problem.
MC tries to minimize this by using PHPQuery to manipulate the output of every extension to bring it more inline with the MC layout and requirements. If it doesn't catch everything, we can use custom PHPQuery injections to fix, and also custom CSS to tweak. This is why this is in alpha testing, because we hve to develop these overrides for extensions that need them. Things that follow the stock extensions like com_banners, and com_weblinks very closely will just work, but the more you veer from that and do your own stuff, the potential from problems increase. K2 for example had a custom 'dashboard' that needed a little work, but aside from that, most of the other sections worked fine. A few things had to be tweaked so they didn't run outside the tighter confines of MC