Lojban is often considered unusuable by humans because of its design.
A better example however, may be languages that have features that humans simply can not use. For example, Elkaril has an odd grammatical feature where precise measurements can be given by how long a consonant is held, and mentions that the 'infinite space' in-between vowels can also be used to encode precise information, though I can't seem to recall what purpose that serves. Obviously, humans couldn't possibly use a language like this, and that was intentional.
Personally, I limit myself to features natlangs have. Not that I try to create naturalisitic conlangs, I just do that because natlangs are of course tried and tested beyond what any conlang could be, so I can be certain that the features they have are something a human could actually use. I used to have a problem with creating weird languages that were clearly unusable by humans.
Human minds work a specific way, and despite our diversity there ARE universals (for example, all languages have nouns and verbs, though otherwise the list of parts of speech within a language can vary widely). So there are limits to what ways a language can work. You may have some wiggle room, but you can't just make up whatever you can imagine and expect it to be usable.