Hi Quiro
the MP3 is sealed by default on all metal connections, BUT the screws and the cable is not water resistent. So the cable needs to have a U turn (so all water pouring down drops on the U-turn down and not into the cable=> controller. As I had to change my MP3 controllers (I have two running) one time I did use simple silicon glue after reassembling.
Despite your reading I heard a lot of Overheating problems, mostly due to "full-throttle" configs (48V+ Batteries, 30A+ config) which get's the max out, but exaggerates the waste heat. When it's hot as well the controller surely died a few times under these overdrive conditions.
In my worst case (EU regulation) I have a 24V/12A config and never even felt the controller nor the motor itself getting warm at all.
Cheers
Sa,m