I've had maybe a dozen JDs garden tractors since 1972 and a 3720 for a couple years and since the last 15 years or so l with plastic hoods and fenders on everything and never had a problem cracking or anything. Although when they came out I had misgiving also. My concern with metal, and maybe they took care of that, was rust and noise. Plastic won't rust and is generally quieter, in my humble opinion.
I weigh about 280 after breakfast, and never had a problem with plastic fenders or hoods from stress, and I don't baby them much, but I try to keep them pretty. I know my Duramax has plastic fenders. I can see the left overs on a tree. I think a metal fender would have to be replaced to.
I did drop a 5" diameter rock on my hood over the top of the bucket from about 3 ft. higher than the hood. After I took a deep breathe and opened my eyes I couldn't find a ding. If there was anything it was a speck. Maybe it wouldn't hurt a metal one either, I don't know. Maybe I was lucky.
If you are locked in to your new buy, don't worry about it. Just get it. If you are not locked in, just stew on it for awhile. I have misgivings all the time to. You will find all kinds of people on this forum that really make you doubt yourself.
I got a Deere because I always worked there and know how much they push quality. They seem to be focused on the ergonomic operation of their equipment, plus the resale value has been great, they tell me, until you want to trade it in.

But anything can break, in time. It doesn't matter what color it is.