So Crystal Assassin Set is a curious story. It was originally meant to be a vanity set. It was just going to be a neat outfit people could use if they wanted, but we realized people would want Queen Slime to have a bit more, so we discussed implementing it as a true equipment drop. We had some planned special set bonuses for it, but they didn't pan out for logistical issues, and we ended up in a bind . . . with release approaching, time running out, and higher-level programmer time entirely dedicated to other concerns, we needed something viable for this slot.
With limited options available, I thought about how people had always wanted a true generalist set, and with that proposal, we went ahead with it. Its not flashy. Its not particularly exciting. But it is, in its own way, unique, and something people have expressed an interest in before, even if its not "meta". It even had a sort of thematic symmetry with the new Ninja set, which also had the same approach.
However, time was relatively tight with it, and we didn't have a ton of time to balance it, and its not set in stone. I've seen some of the discussions for more potent "hybrid class" set bonuses, and I think there is a possibility of that. So that's something on my brainstorm list as well.
tl;dr - I'd rather we got the generalist armor we got than just a vanity set. Its harder to get a vanity turned into an armor later, than it is to buff a weak armor.