I think yes the symbolism of the rose being positive and the briar being negative is obvious. 'Thorny issues' is still part of common speech meaning something difficult, 'a rose between two thorns', etc. In at least some of the many ballads that use the rose entwining briar motif one of the two lovers has been bad and the other good (Barbara Allen, though it is likely an add on to this particular ballad). Of course the best example of single negative is 'Bridgwater Merchant/Murdered Servantman/Bruton Town/Bramble Briar'. I spent several days last week at my sister-in-laws newly aquired mansion trying to pull out entangled 3-metre long briars, rose and bramble, all overgrown into the trees. Fell over twice, 2 dodgy eyes and covered in scratches. I'll have a look in Wimberley and Fleming Andersen. I would say if you wanted to look for likely origins, the Scandinavian ancestors of many of them might hold the answer.

Hmmm who said that? Do you think you might reading too much into it?