Translation:The second text is more difficult but, on the other hand, short.
Can "зато" be translated as "Even though" or "although". This is how the sentence makes sense to me: The second text is more complex even though it is short
I think 'although' is closer to «хотя́».
There's a subtle difference in meaning between «хотя́» and «зато́». «Зато́» usually introduces a positive thing that is in contrast with the initial message. A possible translation is 'to make up for it/this': 'The second text is more difficult but, to make up for this, short.'
Is it wrong to translate 'зато' only with 'but'. It was suggested as a translation for the word, but using only this in the entire translation was marked as wrong.
Unsure of how free a translation was required/allowed, I wrote "the second text is more difficult, on the other hand it is short" (normally I'd have used a semicolon, but DL ignores punctuation). I wasn't sure if I'd get dinged for inserting "but", and the sentence works without it, so I didn't. It should be optional - no "but"s about it. ))
Who uses comparative in one clause and not in the other? This is a gotcha question. Disgusting.
There's nothing disgusting at all about that (educated native English speaker here). It's perfectly OK.
However, I'd be strongly inclined to say "...it's short", because the main verb ("is") does not distribute well across a "but" conjunction with an added parenthetical clause.
However, the given text should be accepted. It's awkward, but literal, and shows comprehension of the Russian.