[jeffbee]

m.2 cards can sink a pretty surprising amount of heat through the connector and the standoff. But case makers can also provision for cooling m.2 cards. Components on m.2 modules are permitted to be 1.2-1.5mm high. My Intel NUC cools the m.2 device through a flexible thermal interface to the system's main cooler (that also cools the CPU). This works extremely well.

One problem with m.2 is the spec would permit a module with the controller on the bottom where it would be difficult to cool. But I think so far nobody has been dumb enough to ship one.