In general the trouble with Pirates is unpredictability. They are very socially liberal, but economically and geopolitically they attract a wide mix of opinions. The have a higher incidence of conspiration theorist, they have a strong anti-US base, and in general they avoid to address these topics since it cannot pass the internal voting that is part of their control structure. In communal politics it does not have so severe implications, so the biggest question is how fit he is managerial wise.
The new mayor was virtually unknown before the election, it's just a gamble at this moment and everybody is "let's wait and see, can't be worse than the previous arrogant mayor Krnacova from ANO"