Polymer - as in nylon reinforced fiberglass - can have a lower melting point. It can also have higher impact resistance compared to other materials, specifically the material used in the original triangular and round handguards. Those were formulated with high heat resistance because of full auto or burst fire.
The video showing a M4 under continuous lock and load full auto fire shows that after a number of magazines, handguards will catch on fire. I suspect polymer handguards would do so earlier.
Since most users will be shooting semi auto, the harshest use would be in competition or carbine class. In that case, I would use milspec. Polymers have been known to melt when used in those circumstances.
For a weekend range and hunting rifle, not so much. Five 30 round mag dumps aren't a constant concern for many. Polymer will do, especially for older clumsy hunters with a bad leg that hangs up on brush causing him to fall down once or twice a day. Younger users will find less cracking and chipping of corners on rocks practicing three second rushes.
Now that you can't tell which I am, I'll say I bought polymer handguards for my AR build, first because it's a hunting rifle, and second, I could get the color I wanted. Should I ever need to burn up magazine after magazine of ammo, I won't be worrying about the guards melting as the impending click of being out of ammo will be more on my mind. And having chipped up some nice wood stocks in my time, I don't want a lot of cracks in them.
Not that I fall down or anything.