As people have noted, there is a fair amount of RNG in the upcoming expansion. I believe the reason to be is because tabletop dungeon crawler games (such as D&D) have a lot of randomness in them. The upcoming expansion is based on this style of game, and as result reflects some of that randomness.

In D&D, a lot of the encounters, loot, and enemies, are randomly determined by literally rolling dice. As someone who plays this style of game regularly, I appreciate them representing that in the set. However, I can understand why those who do not have an appreciation for random dungeon encounters do not like this.

Just thought I would try and offer an explanation as to why there is so much randomness

EDIT 1: People are saying that this makes no sense as there has always been randomness, but the amount of it is what I'm addressing. There is clearly a larger amount in this expansion than others. Rivaling GvG