No question it's prejudice, igor. I actually own a plastic carburetor - my backup mower, which I keep because its engine is interchangeable with my everyday mower, is a Briggs with a Minion (plastic) carburetor. It works properly, too, but they have a poor reputation, even with Briggs.
So far as built-up crankshafts are concerned, they have been used successfully in motorcycles and in light duty cheap engines such as full crank Victa 2-strokes, but those engines are difficult to overhaul. If you rebuild the big-end bearing, reassembling the crankshaft to original factory standards of accuracy is unlikely to happen. When it comes to cantilevered crank (half-crank) engines, they are not so much difficult to overhaul, as likely to require that attention far too often.
I just don't like throwaway engines or major engine parts. They are often good business for the manufacturer, but irritating to hobbyists and collectors.
