I haven't played in about 6 months so forgive me if new content dropped that changes this, but

The difficulty spike comes from pushing endless missions as far as your team skill and composition possibly can. It can get truly hectic once the scaling really starts to ramp up.

The problem is your looking at waiting up to an hour waiting for that scaling to start ramping up depending on your chosen map etc. I wish there was a system similar to Diablo 3's greater rifts or something, that way we can choose the difficulty level the content starts off at, instead of having to wait for scaling to take place.

That said though the content is worth waiting for if your looking to fight some legitimately challenging content.