Engines,
One of the things that they did not think of in terms of V-22 STO was the nose gear design, which had some problems on less-than-smooth surfaces.
Clearly, STOVL has advantages over VTOL. I am not sure in what circumstances the SL bit would help the V-22. Surely that only applies with a larger payload than I can VL with, which I think is quite a lot of payload and means a short range. Reading the report on the STO trials, it looks as if they cleared SL so that they could make repeated STOs at high weights without dumping fuel for VL and then refueling.
Also, note that it was Mr Boffin, who does seem to know whereof he speaks, who raised most of the issues about SL...