I've only failed this once, which was today before reset. The group insisted they were playing properly despite being decked out warriors with full exotic/ascended gear, and my alt thief was left to try and save them one at a time.
Then I got kicked for telling them my warrior who had half the gear they had never failed this event before lol, nor any of my other characters.
I know what you mean, but i find it very hard to distinguish what makes certain groups im in fail. It must be some hidden variable like build or gear. There needs to be better ways to communicate these things to other people. When i run with people i know, mobs die so fast and so do the assassins. When i run with pugs, the pugs die and it's just a fiasco. I also find that groups with 1 or more Guardians or Warriors have a much easier time even if with pugs. Something is wrong with this game's difficulty, scaling, or encounter design....also class balance, but i just can't put my finger on it. I think the thing that is most frustrating about it is, it's really intangible, the reason, as to why some groups fail and others cheese the run. The lack of set roles makes it harder to determine who exactly is screwing up which can be good or bad, im not sure. Anyways, these thoughts befuddle me, but you have my sympathy.
Too hard for Warriors to swap out one utility slot for Throw Bolas, apparently.
GS 4 alone ensures the Assassin never reaches Magg. Assassins start out walking, GS 4 makes them crawl. If you can't kill an assassin in 20 seconds as a warrior, you are doing something wrong. Even full white warrior can do it.