I agree with @PierreB here. Control is a big issue. Though other issues are also hard. For instance, after being able to mount your kite on a sailboat, you will face issues with safety, handling low and gusty wind, wear, reliability, high wind etc etc. Having solved the sunny days is a really good start and a big step. But its not like once it works, the work is done.
Also saying controls is the only issue masks that quite a few of the efforts that are and have been are based on ideas that are unlikely to work and maybe could be proven impossible except assuming groundbreaking new technology.
I think there is a difference in saying this would work if we solved some issues (maybe fusion energy) and saying this would work if we could reduce the mass by 75% compared to the lighest materials currently available (mayne Makani), and sound is not an issue, and it can be made much cheaper, and so on. Those kind of advances are just quite unlikely.
I think most things on that list (electric vehicles, brain computer interfaces, cripr) did not have that amount of magic needed. Just money + talent