I cant speak for the MKIII, but ive had it on my 60D for some time now, and the reward has outweighed the risk for me. Its an awesome piece of software. I think the disclaimer is basically to avoid any law suits just in case someone ever did brick their camera. You know how people are....someone might try to modify the code themselves or something and then screw it up and try to place the blame on A1ex. Anyhow, One thing i've done is to make sure and just follow the protocol ML gives you when installing the software precisely to avoid any glitches. (Not to say there are any) but I do what they say. And when I dump my ML card to a hard drive, always be careful not to delete any of the ML files on the card when deleting video's or images. And I always use the "stop devices" thing in windows before removing the card just to be safe.
When I was reading about ML° before installing it, there were thousands of people who had installed it successfully on all the cameras they wrote it for with not one actual case of it killing a camera. Which I thought to be pretty good.
As far as overheating goes, I might be weary of that since Canon diddint necessarily design the camera to record at a higher bitrate, and/or for hours on end, but thats only if you override it. I never find myself shooting for hours straight as it is, but some might. For instance Astrophotography mods:
http://glogg.jupiter-io.net …ames/350D/cooler_350d.jpg
I tried cranking the bitrate up with ML to like 3X once, but the recording stopped, so I diddint really mess with it again. There are some really awesome features in ML that I would even pay for. It really makes the DSLR more of a video camera. Since the MK3 is newer I would probably research any issues (actual issues, not exaggerated ones) found by people who have installed it thus far. Been using on my 60 for over a year now with no glitches. I have to say I was scared the first time installing on a brand new camera 
