I would agree that the MU-2 won't likely make history as one of the finest aircraft of all time. I have 900 hours in MU-2Fs and had more trouble in those than any of my other 16,000 hours.
Door seals not deflating, gear needing to be cranked down - yup, as mentioned earlier, had that happen too.
Also the tip tanks had an electrical valve that would occasionally freeze up preventing 90 gallons per side (as I recall) from getting to the mains and therefore the engines. A later fix from Mitsubishi installed a heater.
Also the Garretts are hard on batteries, and those nicads were prone to thermal runaway if you weren't careful, and didn't use ground power units frequently. With the aft battery sitting over the control cables for the tail surfaces, this was definitely not a good thing as at least one MU-2 operator discovered.
The main gear doors opening during the retraction cycle acted like clam-shell buckets and could make a take-off from the likes of Denver rather interesting.
Having said all that, the aircraft is very strong, stops great on a short, slippery Canadian runway and offers excellent training for jet transports. Just treat it VERY carefully and get good training.