That's what liberals say about all new military technologies -- "fundamentally unsound". They confuse what is capable in theory with what has been implemented in practice, and never give the engineers the time to work out the problems.
The Osprey design is a very old and very well-known engineering problem that many have failed to solve. In theory, the engineering problem can be solved and doing so would have huge military advantages, and the DoD has the patience and money to see that it actually does get solved. That the Osprey works at all is a testament to the engineers who were able to solve a number of longstanding theoretical problems surrounding those types of designs.
This is no different than ABM systems and hyperkinetic rocket motors. People dismiss them because they are buggy and unreliable for decades, but once the systems finally come together people wonder how our military functioned without them. Difficult engineering rarely works perfectly out of the box, and some problems will never be discovered until put into use, something true of just about everything.