Extremely unprofessional. They will string you along for months without any updates or providing any sort of closure. Followups don't help either. One can only "assume" that one has been rejected. After the final round, if they decide to reject you, don't expect a word from them. Extremely disrespectful to the candidate.
Average interview process. First round has you record your responses to a set of questions with your webcam. Second round is a code review (Python or C++ only). Third round is 3 1-hour interviews with the team.