It totally depends on the quality. There are some polymer mags that are rock solid, such as the Bulgarian circle 10 mags and the Russian Bakelites for the AK and the Magpul P-mags for the AR; and then there are cheap pieces of crap, like the Tapco and Promag polymer AK mags, and the Israeli plastic mags for the AR. The same goes for metal mags. The main thing is to research and buy a mag with a reputation for reliability.