There are a few factors here. The main point of course is, how you treat the disks. For the mobile ones it is more likely that they get stressed (like dropping while operation or getting frozen or grilled in the car).
Unless you have specially small portable devices they typically contain the same disks. The external enclosures not always have fans which can lead to heat problems. On the other hand desktops have more parts generating even more heat (like power supply, CPU and GPU).
So it really depends on your devices if they life longer or shorter. It also depends on how often you (un)plug them (compared to switching desktop off or on).
The main rule here is: the device will fail in the moment you least expect it.