Always disenchant nerfed legendarys.
2 possible outcomes:
1- The card remain strong and playable after meta stable again, in this case you can craft again for the same dust.
2- The card don't be playable anymore and you lose the chance for disenchant for full dust value.
There is no disavantage in 1, only in 2.
3- You now have the opportunity to open it again in packs.