^thisI was issued a Glock 17 for several years and I also personally owned a Glock 21. I fired thousands of rounds through each and never thought about what the rod was made of.
It is just a guide for the spring after all. I have 1911 pistols with no guide rod. They work with no malfunctions too.
Believe it or not, gun designers know what they are doing. If plastic was not good enough, they would use something else.