There's a lot of subtext to cheap (at least in my experience), particularly in regards to quality. If someone wanted to say that something did not cost much without saying it was of poor quality, they might prefer 'inexpensive' to 'cheap'.
Personally, I tried out 'thrifty' as an alternative. It was not accepted, but I have reported it.
Yea, their speech is sometimes difficult to understand properly. You can always of course report it if it gives you trouble, and perhaps it will eventually be reviewed if enough people do so. However, in the real world, we don't always hear or pronounce things perfectly (even native speakers, not to mention different dialects/accents may affect pronunciation). It's almost like having to do a double translation. For example: maybe it sounded like 'también' but también by itself doesn't make sense there, it needs a verb to modify. So you think to yourself what verb might sound like también, and you come up with 'tienen' which makes a valid sentence when added to the other words (even though it wasn't what Duo was looking for).
For a little perspective and perhaps amusement, I recommend trying the English from Spanish lessons on Duo and hearing what some of the pronunciations sound like. For example, the male voice always stutters over the word 'milk' and the word 'close' is always pronounced like the close in 'It is close to here' but never as in 'please close the door' no matter whether it's being used as an adjective or a verb.