Chaos units with deadly cold are sufficient in general.
But this is, again, my opinion, and I consider it a plus. The more interaction options, the better. In the current balance, fighting machines is a possible option. But... there are potential risks. At the very least, Chaos units lose an important ability when staying in a castle: the ability to use cunning. As a result, destroying a catapult and engaging in a battle in the garrison becomes not the most successful choice.
For non-Chaos factions, Chaos is generally a rarer school of magic, and choosing it involves a certain risk. Also, it's not a given that your opponent will use a hero specializing in machines. Even if you see him in the tavern, the enemy might use him for breaking through defenses and send a different hero to the final battle.
In short, all of this is quite complex.