It is not uncommon for HVAC equipment to be located in an attic. Generally, that attic is not a conditioned space, so humidity levels may be high. If high humidity is present, there is a great danger of having condensation from the A/C equipment. This can lead to insulation damage, collapse of ceiling drywall below, and mold.
To avoid these problems, it is important to maintain a vapor seal between the cold surfaces and the attic air. Do this by installing a vapor barrier on the outside of the duct and equipment insulation.