Excerpt from wikipedia:
Safety concerns
Concerns have been raised about safety; there have been 337 fatalities from MU-2 crashes.[11] As of October 2005, the Federal Aviation Administration (FAA) has begun a safety evaluation of the aircraft and decided that the aircraft has met its certification requirements - it is safe when operated by properly trained pilots who operate properly maintained aircraft. The FAA is in the process of mandating training specific to the MU-2 as it has in the past for other aircraft. When such mandated training was required outside of the U.S. the MU-2 accident record was vastly improved.
Because the MU-2 offers very high performance at a relatively low cost, some of its operators lack sufficient training and experience for such an advanced aircraft. The MU-2 has performance similar to a small jet, yet early pilot licensing required only a simple endorsement from much slower twin piston-engined licenses. The large number of MU-2 pilots who were inexperienced at high speeds and high altitudes seems to be the cause of the high crash rate. Once a type certification was required for an MU-2 license the accident rate dropped to normal levels.
A design feature of the MU-2 is its high cruise speed while having a low landing speed. This is accomplished by using full-span, double-slotted flaps on the trailing edge of the wing. These flaps give the MU-2 a wing area comparable to a Beech King Air in landing configuration while having a wing area comparable to a light jet while in cruise mode. The full-span flaps meant that over-wing spoilers were employed instead of conventional ailerons. These spoilers are highly effective, even when the MU-2 wing is stalled. Some fatal accidents have occurred because normal engine-out procedures for light twin aircraft are not effective when flying the MU-2. The commonly taught procedure of reducing flap following an engine failure on take off leads to a critical reduction in lift in the MU-2 due to the highly effective double-slotted flaps. When pilots were taught to retain take-off flap and to reduce climb rate in the event of an engine failure, MU-2 accident rates were reduced to almost nil.
From an FAA press release:
The FAA began an aggressive safety evaluation in July 2005. The evaluation is performing a detailed review of accidents, incidents, airworthiness directives, service difficulty reports, safety recommendations and safety reports. It also is examining pilot training requirements, the history of the aircraft's commercial operators and possible engine problems. The goal is to identify the root causes of MU-2 accidents and incidents and determine what, if any, additional safety actions are needed.
In early 2008, the FAA issued a Special Federal Air Regulation (SFAR) directed at MU-2B operations. Pilots flying this aircraft after that date (current MU-2 pilots would have a year to come into compliance) were required to receive type-specific initial training, as well as recurrent training. It also required that a fully functional autopilot be available for single-pilot operations, and that FAA-approved checklists and operating manuals be on board at all times. Also unusual for this SFAR, pilot experience in other aircraft types cannot be used to comply with MU-2 operational requirements - for instance, the requirement to perform landings within the preceding 90 calendar days before carrying passengers is altered by this SFAR to require those landings be made in the MU-2.[12]
As of March 29, 2016 there have been 3 fatal accidents involving the MU-2 since the FAA SFAR (Title 14; Part 91, SFAR Number 108) was implemented.