I make a device akin to a guitar pedal. If you want to use a plastic case, it has to be designed for mechanical ruggedness and protection of the innards. For small time pedal makers, the cost of design and mold tooling rules this out, though I wouldn't try to predict the future of technologies such as 3d printing.

But if you're limited to drilling some holes in an off-the-shelf enclosure, it had better be metallic. I learned that lesson the hard way, and I now use metal boxes exclusively. Luckily for me, my product somehow hasn't attracted the interest of Behringer or other low-cost makers -- yet.

Also, judging from some returns I've gotten, gadgets can be subjected to a lot of abuse. A musician can control this, but a gear maker can't.

I've seen the insides of some early Behringer gear. A lot of their early stuff was mechanically delicate, and showed signs of extensive "re-work" in production.