Yes having disadvantage kills it even if you also have advantage but there are many ways to improve hit chance
That actually seems to be wrong. I thought the same and I filed a bug report to Larian, when my Astarion had disadvantage from heat metal yet had advantage from obscured by shadows and I was able to make a sneak attack against an enemy within 1.5m from my ally. Raze responded and said it was working as intended, so the "can't have disadvantage" part is related only to the 1.5m ally condition.
