I want to start off by saying that Nexus Blitz is loads of fun. It's probably the single best thing that has ever happened to league (yes, even better than URF).
However, there are some insane balance issues here. And I'm not talking about certain champions being OP (although some are) or events appearing in the enemies territory (even though they do).
The big balance issues seem to be the huge disparity in the event rewards. Plus rift herald.
Let's start with Rift Herald. That thing alone pretty much guarantees a won game. You get all the power of Herald, PLUS all the power of Baron, on a confined map where you crash two lanes of minions into a single tower. It's WAY too strong.
An event can start, and a team can prioritize Herald while the enemy team prioritizes the event, and the game becomes entirely RNG. If the event reward is cursed enemy minions, the herald is useless (no minions to buff) and the team that took Herald probably just lost the game. If the event reward is BF shields, Herald doesn't matter much (enemy team can just crush you in a teamfight and then kill Herald), so Herald's team loses the game again.. If the event reward is GAs, Herald is probably an even trade.
If the event reward is Ocean drake, Blitz Hooks, or Catapult of Champions - the team that took Herald probably just won the game.
Since you don't know the reward, the RNG is insane.
Likewise then goes with the event rewards, even not considering Herald.
You can win the first two events and get Blitz Hooks and Catapult of champions and be pushing a lead, but you lose the third event and the game off of BF Shields.
I like that the rewards have variety, but the power level should really be made more level across them.