Also keep in mind that they may expect a certain amount of failure. If the test is too easy and all they get back is successful apps, they haven’t learned anything about the different applicants. I’ve heard a few stories of people that didn’t do well on these tests but still got hired because they knew the test was hard and just wanted to see how people handled it.
If nothing else, you learned some things. I’ve done about 5 of these things now and haven’t been hired. On a few of them, I definitely got the “Oh crap, I’m way over my head” feeling. But I learned something on each one.
I might suggest building the app anyway. It would be a nice portfolio piece and it would be a chance to fill in some knowledge gaps. Heck - it sounds interesting - I might build one myself.