Engines,
I don't think that there is anything with the B that can't be worked through, although the DOT&E has a long list of squawks...
But... When everything is fixed to meet KPPs, what will the price tag, acquisition and O&S (which is not a KPP), be? And what will it cost to add any growth capability?
Having been pretty close to what was happening in 1994-96, what I saw was (1) DARPA CALF designs (STOVLs, with CTOL versions created by replacing the lift fan with a fuel tank) being adapted to add CV and (2) Boeing's design, which was an outside push into CALF, but which was clearly STOVL-based with its front engine. Clearly the USAF assumed that one engine would cost less than two, but we all know how to spell "assume".
V-22? What I saw was a Marine program for a helicopter called HXM (to which spec the Boeing 360 was designed) being rolled into JVX... The Marines were always the biggest customer and the one where the JVX was most important. The Army was looking at casevac, never logistics.
Good point: RVL will reduce the ground effect. But don't you still need 3,000 feet for the KC-130Js?