The main advantage of polygonal rifling is that the gas seals better behind the bullet, creating muzzle velocities that are slightly higher and more consistent than traditional rifling. However, because the gas seals better behind the bullet, if there is a build-up of lead in the barrel you're more likely to create an over-pressure situation.
Glock specifically had a few lawsuits from barrels blowing up when people shot lead bullets and didn't clean the lead residue from the barrel often enough, so they caution heavily against it. But it's fine to shoot lead bullets in a polygonal barrel as long as you clean it often enough that lead doesn't build up.
0331: "Accuracy by volume."