Yes, we have statistics on events. Those have been really important to us in fixing events, if we see an event that never triggers for anyone then it's almost certainly broken! There are definitely rare events, usually requiring some other chains. For example if you let Pieface try his hand at medicine (in itself not too common), there are some followups. There are rare diplomacy events that require something unusual, like a tribe that's militarily stronger than you. Some others are rare because they require non-typical situations such as building a Chariot when you have the Circus Maximus, or a new leader to take the throne who is unmarried but has a lover at the time of coronation.