Why do people not recommend fiberglass for body replacements in this car?

Maybe it's because I drive a 2012 Honda Accord as a daily right now but I've become completely accustomed to fiberglass.

Is it a "You can do it but it's not recommended because the steel body is superior," kind of things?

I'd like to at least think that it can absorb shock a bit more than the steel could, not that I plan to get in any accidents.