An attic area needs to be VENTILATED properly in order to dispel any heat build up within it. So the problem could be there is NOT enough soffit or ridge venting. For every 300 sqft of attic floor area there should be a minimum of 1 sq ft of venting ,divided equally (50/50) between the soffit and ridge vents. The more vent area the better.(you can never over ventilate an attic as long as there is proper insulation in the right places)
As long as any ductwork is adequately insulated and also the attic's FLOOR area is well insulated along with proper soffit/ridge venting, there should never be any ice build-up or jams on the roof.
For all it's worth (imo) installing heating or coolingductwork,etc in an attic space is certainly NOT the best choice, but if it does become absolutely necessary then as long as the PROPER INSULATION and VENTING are in place then it may be somewhat ok.
Imo, the basement is still the most logical area for hvaC equipment.