I find many mac models have conformal coating on the logic boards. This can also make unsoldering more difficult. And if you are soldering/desoldering anything attached to the ground plane, you need a soldering iron that can source a lot of heat.
In summary:
ground plane connected part,
conformal coating,
RoHS (I think most intel iMacs are RoHS compliant),
means you need something more than the standard pencil type soldering iron. I use an old Weller WTCPN, which is a beast of an iron. The power supply weighs about 8 pounds.