Prize fight, as an example, is a cool idea and a lot of fun, but I hate that you can go in to a 2v1 and feed kills hard because you blew your 100 second CD 30 seconds before you realized Prize fight was coming up. Also the fact that you get killed a second before the Bardle Royale but because you die to the ignite two seconds after them, the Bardle Royale is now a 4v5.
It really sucks because it incentives you to spend the last 30 seconds either not fighting or farming jungle mobs for a quick item before the event. And yeah, I get that it's the strategy of the mode but man does it feel shitty. I personally think it'd be better for Nexus Blitz events to reset all CDs and health pools right as the event begins, so as to put everyone on an even footing. (And maybe even encourage you to keep fighting just before the event.)
Oh yeah: **TURN OFF NEUTRAL MOBS FOR THESE MODES**. Hate that people can totally circumvent the difficulty of buff/scuttle fights when you can just mash Q and turn Golem into pebbles. Plus the absolute cheese of grabbing herald and sending it down a lane during Bardle Royale so the enemy can't actually deal with it.