I have about half-a-dozen Behringer mixers ranging from their tiny 5-channel to their 12-channel and everything in between. I think they're great. We use them in my studio mostly for headphone mixes but we also use one for our playback monitors. I haven't had a single failure with one and they are extremely quiet. I don't know why they are so cheap but it is probably partially because you are not paying for some big name. In fact, the name you are paying for is known for inexpensive audio gear. Plus, they probably make a higher quantity of these mixers which will generally drive the prices down.
In terms of quality for the price, it probably just comes down to the fact (as I see it) that good audio circuitry and parts are just not expensive anymore. A $3 potentiameter might be as good as a $20 one from 10 years go. That's just a guesstimate, of course. Think about how much CD-ROM burners cost 10 years go. $500 to $1000. Now, you can get a better one for $50. Audio technology improves over time just as everything else does.
I also use one of their dual channel tube pre-amps and a 4-channel direct box and they seem to be very well made, sturdy, quiet, and reliable. Sure, I could pay more do better. You can always do better. You can always pay more and get better sound and quality but if you can't tell the difference between an $80 Behringher and one twice the price, why pay more?
Note that I haven't done any side-by-side A-B tests or anything like that. When I got my first behringer, I did some recordings with and without it and I didn't notice any significant difference in the noise level between the two. They seem to have a lot of headroom too. If they do impact the sound in a negative way, I haven't noticed it.
We have a Leo's Pro Audio in Oakland, California. Many times I've perused their mixers and their salespeople have never tried to upsell me to a Mackie or anything else. This is a place that sells $2000 microphone pre-amps and $3000 microphones. I'm sure that if they had a negative opinion of Behringer gear, they would use it to try to make more money.
It would be interesting for one of us to do a test by recording an audio signal direct and one through a Behringer mixer and have a 3rd party do a blind A-B test.