I have a couple of externals that have lasted me a number of years, though I haven't used them for games. Couple of questions for you:
How long do you have them on for a day? Whats the ventilation system around them look like? Are they stacked on top of the system? Is there at least a foot in 3 directions of open space around them and the system to allow for airflow? Are they in direct line of the Wii U fans?
All of these things will affect longevity.








