The temperature problems and and resistance to fuel/oil were the first things I worried about... Presumably the designers have also considered them
Have a look how much plastic is under the bonnet of a modern car. At a lecture in the late 1980s a Rover development engineer said that the now infamous K series engine was actually their second choice. The one the development people wanted to put in production had, if I remember correctly, a phenolic resin crankcase.
Some Husqvarna chainsaws have plastic crankcases, and tens of thousands of Testors 0.049 model aircraft engines did too.