They are polarized, but the only "bad" thing about reversing the polarity is, the hot side gets cold and vice-versa. So you can use one side of a peltier as a heater or cooler by simply reversing the polarity.

What happens if you reverse polarity on a Peltier?

Reversing the polarity will switch which side is hot and which is cold, so you could, for example, heat & cool the inside of a box with one Peltier. A simple DPDT switch could be use for this, or if you prefer to do it electronically, a H-Bridge would work.

Are Peltier elements reversible?

Joule heating, the heat that is generated whenever a current is passed through a conductive material, is not generally termed a thermoelectric effect. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not.