Very pleasant and efficient interview in general. We had an initial screening stage with very generic technical questions and a second stage with a take home assignment with pretty generic science concerns. One problem is intentionally leaving everithing too underdefined and generic, even though the interviwer always had one specific answer he wanted to hear. Thus, any answer could be correct (but from the interviwer perspective, they're all incorrect, given that he wants to hear his version). Another problem I found is the lack of criteria for evaluating the assignment (same reason as stated above), and the lack of feedback. Given that the interviewee spent a bunch of hours, a single feedback on what went wrong is not much to ask.

The mechanics were as follows: I had an interview with my now manager, and then a one week data science assignment solving an interesting problem. Then I had an interview with more members of the technology team to present my solution to the assingment, followed by an interview with the office manager and then the CEO. It was clear in the whole process that the Wesper team values many things: obviously technical knowledge and capability are paramount, but the human side of this purpose driven company is very important as well.