What is disadvantage to lay power cable in the duct?

If you lay a power cable in a duct, and it overheats due to a short or overload, the smoke from the cable burning will fill the duct and be transported by the duct to places it should not be.

If the cable is not balanced, i.e. not paired up with its neutral return, current in the cable will induce a current in the duct via electromagnetic induction, causing heat, loss of power, and potentially fire, even if the original circuit was operating normally.

The latter effect is why penetrations for single cables in distribution panels must be slotted, so as to break the electromagnetic field.