Personally I've used several different weaponn DDs in legion, including 95 Rag, 95 Apoc, 90 Ukon, 99 h2h relic, 99 relic RNG, 99 Masa, 99 amano, OAT GS etc. And in general, Rag(not even 99 rag, just 95) has highest performance on top of able to 2hr zerg.
I did not ever once say that if you don't have X weapon you can't clear, there are groups who cleared without them yes. But you can't deny that certain weapon pulls way ahead of another, which is mainly because of enemy crit-hit%- and high evasion+def I believe, which just hurts crit-hit WS job or weapons without acc on them. If you do other event such as ADL/Nyzul/VW T3, the difference between every DD job/weapon wasn't even this big.
I fail to see why enemy crit-hit%- and that high evasion/def is needed in this event, it simply makes certain weapon stand out and that's it. It doesn't suddenly make this event "hard", or your DD suddenly more "elite"(a 95 rag isn't more elite than other weapon no?), it just make the gap between DD jobs even bigger.
As for legion specific gear set, generally I will ask SCH to have macc stun set+Impact(ideally need 500+ enhancing/healing/nuking too, but I'd be happy enough if they can stun), DDs for PDT- MDT- set, and will ask them to tweak TP set for more accuracy. I used to ask SMN to have capped or close to capped summoning magic skill too, but I don't really care anymore as SCH is the one stunning mostly and now I just ask for capped bloodpact delay-. For those ppl who is used to lower tier VW, stun set with macc, PDT-MDT- set and accuracy for DD is simply not needed. A lot of SMN also don't use Ramuh outside of legion. You can argue that PDT- MDT- set and accuracy every DD should have and not just legion DDs, doesn't mean everyone would be bother with them when you don't really need those in VW with fana/fool/temp.
And yes other event you'd still want "good gear", but it's not as hit or miss as in legion, thus more acceptable to use less optimal set. In VW a pt with "good DD" and "average DD" is the difference between 2 min kill and 5 min kill, but still kill regardless. In legion it's between clear wave 3 and get Gallu/Rex drop or no Gallu/Rex drop at all.