From what I remember of 2nd gen, quads stop being good partially because human plus still exists (this is largely true of 1st gen too), and that they lose the one of the niches that they had in 1st gen. Their ground speed being very comparable to boost hopping everywhere. It doesn't get better for them in 3rd gen as they've been changed to the sliding movement to normal walking movement, which while it lets quads move at the same speed of most middle weight bipeds, it also makes using their last niche (using cannon weapons while in contact on the ground) nearly impossible to use general combat. Which just makes them effectively middle weight bipeds with and absurd energy cost.

Also the karasawa in another age wasn't that useful

The Karasawa from 2nd gen onwards is for dumpstering AC opponents at mid range, and it keeps getting nerfed nearly every single game from AC2 on through to both early and late 3rd gen.

I actually never really find a build the moonlight blade fits into cos its such a high risk weapon to use.

The Moonlight is really good in 1st and 2nd gen because human plus exists, 3rd gen Moonlight (and well any blade) has the unfortunate problem of being in the gen that introduces left arm held guns which are really good for the AC v AC fights littered throughout 3rd gen. So blades at this point are pretty much the most cost efficient for clearing trash mobs.

Just started AC3 thats gonna be so hype

Btw, have a set of hover legs ready to go. Because AC3 (and 3rd gen in general) is where several missions are purposefully designed to use them to have a significantly easier time.