Just a quick exposition. Comparing it to other 16-bit consoles the stock TG16 is pretty reliable. Now the controllers both the cords and connectors have problems. But the system itself rarely ever fails in my experience.
With that said, I don't see a large number of failed SNES, Neo-Geo or Genesis systems either.
This changes when we go to the other NEC hardware variations. First off the Turbo express. Obviously these have bad caps, and it's an epidemic. During their lifespan they worked. But obviously these haven't lasted that long. And yes, Game Gears do have capacitor problems. But generally it can be narrowed down to the sound board. The TE seems generally to be the least reliable of the handhelds on the market in its era (GameBoy, Lynx, Game Gear).
Next off is the CD add on. Since Nintendo didn't release such a product to the public, we can only compare it to the Sega CD. The two models of Sega CDs have various problems, but it seems like both the model 1 (occasional motor problems) and the model 2 (games lock up periodically) are both more reliable than the Turbo CD.
The final comparison is the Duo. Compared to the CDX (combination Sega CD and Genny), the Duo really isn't very reliable. Compared to the JVC X'eye (Sega CD, Genny and karaoke device) we find a fairly similar system as far as reliability. The X'eye has a bad door sensor button and also has problems with caps and even laser issues.
Overall considering their age we find this. The stock TG16 stacks up well. The express, add-on CD and Duo are below the curve on reliability.