Yea mate i be aware that these two be not the same. Reading what you wrote, you said that many bits stuck together by welding/bonding for unibody. That is pretty much making them into one, henceforth why i said that unibody = onebody. Same goes for monocoque = onebody/oneshell even though way you make them is unsame/not same, since the thought is that these two will be shaped into one.
Indeed, they are both a single structure. The key difference here is that a unibody is a single structure made of multiple separate parts, and a monocoque is a single structure that is itself a single part. Both perform the same function, but they each have their own characteristics that are significant enough to differentiate them. A monocoque is usually lighter and stronger, but more expensive to produce than a unibody. Unibodies are still strong, but are only as strong as whatever bonds their components together. They're cheap and common because each component can just be stamped from a big sheet of metal in one operation, and removed/replaced as needed. If you break a monocoque, it's practically scrap. I realise that BeamNG will likely not make this distinction in game. As far as the jbeam is concerned, each (structural) chassis is a monocoque. So in the context of this game, right now, it's not a big deal... unless someone wants to write jbeam for a unibody chassis that has break points for each discrete panel and box section that makes up the chassis... I somehow doubt that, but it would allow for the breakages mentioned in the OP at least.