You can't just rank the candidates you like, in the order you like them, because your genuine top 5 choices might not include the people who are most likely to win. You'll be "throwing your vote away" unless you include the most-likely-to-win candidates - that is, unless you vote tactically.
I've seen lots of discussion in NYC where people are trying to figure out the right tactical voting to do given their views and given the fact that they can only rank 5 candidates. Ranked choice voting is supposed to make tactical voting essentially irrelevant, so this is disappointing.