After detaching certain lenses and powering off the camera, the camera will lock up once being powered back on. The only way to recover from this is to remove the battery.
Also, when using tele-converters, it is important to follow the right order of attaching converter and lens in order to be able to set the mode of the adapter correctly and to prevent incorrect reporting of focal length and aperture in EXIF.
Neither of these quirks are deal-breakers in my view, as this adapter allows you to attach 24MP APS-C and now 36MP FF cameras to canon glass, neither of which is possible with Canon bodies.
I've heard of these adapters, but have not used them. As a result, I can't comment on their functionality. Metabones and Conurus have collaborated for quite some time now, producing these adapters. In my experience, Metabones' customer service is good, but you have to send your adapter back to the factory in Hong Kong for firmware upgrades (which I think is utter nonsense). I guess it's a case of pick your poison, as usual…