The Mil Mi-12 was a good example of how aircraft concepts can look good on paper, but flawed in practice. Much larger than anything before or since, with fantastic lift capability, this helicopter was so large that it needed an airport to work from. And the practical advantage therefore, in practice, was - nothing. A fixed wing was more practical. They built two prototypes; it never went into production.
If the tilt rotor can't get into helipads, it needs an airport and then......
I just hope they can get some use for the huge amount of money poured into the project.