Free, good libraries, simple
You can’t click the board. It just shows you the problem, and you have a button to show the solution and another button to go to the next problem. You can’t go back to previous problems or explore possible outcomes. The app even says that it expects you to do the work on paper instead of in the app.
I can’t tell if this is a really high IQ tsumego app or lazy programming, but i basically agree with the teaching philosophy, and the library is great, and it’s totally free, so I’m gonna use it.