Whats with the Rare Mob stacks in Heist?

I just failed a Quest Heist (!) on lv 70+, because during the time Huck opened the very first gate after i took the Curio and the alarm went off, the game spawned about 70 mobs on the opposite side of it. Tons of Explode-after-death spider mines and about 20 Rares.

How the hell am i supposed to do anything? Not to mention the loss of frames and it just being a Quest, not some suped up Grand Heist.

What the hell. The demotivation is real.

There is a new skill gem: Summon Shiny Hero. But it must be leveled to 100 first and spiced with alternate quality.