What's the hardest part of running an amateur tournament?

I’m launching tournament99 here on Thursday, November 5: a free tournament maker that comes with the rules of 36 sports built in (points, tiebreakers, what happens when a knockout game ends level).

Before launch I’d love to hear from anyone who has organized a league, an office ping-pong bracket or a club event: what was the most annoying part? Scheduling, settling ties, keeping everyone updated, something else?

I’ll read every answer and it’ll shape what we build next.