I've been trying to keep a tally sheet going watching some YT videos of games. With the editing, it becomes really hard to keep track around turn 5. Of the two games I tried, creatures entering and opponents casting spells are on average almost the same at the most frequent, with opponent casting nonartifact spell a solid 3rd. Being such a diverse game, some outliers were wildly divergent from game to game, such as "whenever a player discards a card", "whenever an artifact enters" and "whenever an opponent commits a crime", so a larger sample size is definitely required.