If it's exactly the same board and layout, an ultra slim laptop motherboard will be designed for different thermal limits than a desktop board. Also a laptop board will have its shape and orientation optimized for a very thin heatpipe + flat blower cooling system, something in a desktop might be different. Take for instance the same CPU used in a gaming laptop or in an Intel NUC/tiny box shaped PC, the heatsink/fan arrangement will be unique between the two. But then again this new imac is so thin it might as well be a laptop.
