Indeed, it is on pure luck.  Sometimes, a single spin can land to a bonus round, and sometimes even with 200  or more spin can't land you a single bonus round.  I had experienced them both.  And what funny is even if we are lucky enough to get a bonus round, we are still unsure whether that bonus round can give us a multiplier.  There are times that I won't hit a single win in a bonus round after hundreds of spin to get into it.