This 'phenomenon' is addressed under 'myths'. Deploy a spoiler and one wing has more lift than the other so the aircraft rolls around it's longitudinal axis (i.e. the wing with more lift goes up), exactly the same as when you use ailerons. Essentially any aircraft that makes several rapid roll inputs will lose performance (in this case 'a bit of altitude' ) all else being equal.
I believe this is manfacurers candy coating. The difference is that with ailerons you have one wing creating more lift and the other creating less lift causing thus not losing any total lift as ailerons are applied. The same control input on a spoiler equiped airplane causes one spoiler to raise on the wing you want to roll to and nothing happens on the other wing. Therefore you are decreasing lift on one wing and it is not replaced by lift on the other causing a net loss of lift.
I'm not sure where you got your data on the MU-2 having a better safety record than other turboprops of its class. I also don't know what makes you think that MU-2 pilots would have less experience than pilots of other simular airplanes either.
In any case he MU 2 is certified in exactly the same way as all other aircraft and will therefore perform as intended following an engine failure with whatever amount of reduced thrust and increased drag the aircraft has, providing it is flown correctly (part of this certification procedure requires that an average pilot can achieve the required performance so it shouldn't be any harder to fly than any other plane)
This may have been true origonally, however due to the accident rate of this airplane the pilots are required more training to fly the MU-2 than other simular airplanes.
In fact MU-2 pilots are required by the FAA to receive type specific training (simular to a type rating) that is very unusual for aiplanes weighing less than 12,500 lbs. The FAA also requires this plane to have an operating autopilot to fly single pilot, another oddity. These requirements are due to the airplane being much less forgiving that most other.
The primary effect of engine failure is yaw, and therefore the correct control input is rudder not aileron (or roll spoiler). In fact once the correct rudder input has been made most aircraft require very little or no aileron input.
This is a little misleading. When an engine fails you not only get a yawing moment, but you also get a rolling moment. You are absolutely correct that the correct control imput should be rudder in this airplane. In most any other twin engine plane you can step on the rudder and keep it mostly steady and control the roll with ailerons. This is what kills MU-2 pilots and this is why the additional training is required.
This airplane is not as tame as the mfg. would like you to believe. Other aiplanes in its class such as a King Air 90 Cessna Conquest, Piper Cheyenne, etc. will kill you alot less quickly than the Deuce.