How old is this information?
Every VOR I've seen here in the UK is an obvious phased array. One has about 30 aerials in a circle some 15-20m dia and one feeds signals to all of these, phase shifted so as to create an interference pattern which forms a beam (or two; I don't know the details but it's probably two or four).
Phased array radars work on the same principle.
VORs with rotating aerials still
AFAIK exist somewhere in the USA.
A TACAN, whether constructed as a phased array or as the old rotating aerial, working as it is on a frequency several times greater than a VOR, would be correspondingly smaller in diameter and that has got to be the #1 advantage. One could put it on the back of a truck.
Both a VOR and a TACAN emit a continuously varying radial so there is no difference in resolution - the res is infinite.
UHF will be a bit less affected by terrain than VHF but I can't see that is relevant since e.g. a localiser is VHF.