In a polymer bottom gun the balance is acceptable at the beginning of a full magazine but as you use rounds the balance changes very rapidly to - top heavy. The more mass on top - the more the gun will rock in your hand. The same thing happens with an all steel gun but to a MUCH lesser degree.
Another 'interesting' comment not addressing the whole truth. The fact is that the most popular polymer pistol, the Glock, produces much less sharp recoil and muzzle flip than a similar weighted steel pistol because of 2 items that the poster ignored.
First, Glock's polymer frame flexes which softens and spreads the recoil impulse while the steel frame doesn't. This reduces both felt recoil and muzzle flip.
Second, Glock's design puts the bore axis much lower than steel framed designs which reduces the muzzle flip because the recoil impulse is more in line with the wrist.
Together, these produce a much lower felt recoil and much less muzzle flip even when down to the final round.