We are talking about 2 different types of "plastic crankcase" here. The fact is, we generally reserve the terminology "metal crankcase" for the models that have a vertically split crank case that also forms the body of the saw. You know, automotive style, with a block (crank case) and the head(s) mounted on that.
So, just to be clear, there are many saws (like the MS250, MS290, etc) that have the crankshaft wholly enclosed in a metal "shell." I suppose they "technically" have a metal crankcase, but since those shells (which are minimal in size and do not have extensive heat sink properties) ride in plastic housings, we also refer to them as having a plastic crankcase. The other type of plastic crankcase saws we have are truly saws that have, at least for half of the crankcase, plastic. A good example of those are the Husky 350 (and it's little brothers) and the many Partner saws from which that beast sprouted.
The 350 seems like a real good saw so, apparently, the whole thing works terrific if well designed.