Current student here.
Speaking on the Culture of Stress:
It exists and that's good and bad. CMU tends to attract students that work extremely hard. I was more laid-back in my undergrad, so seeing how hard people work has helped me a lot in terms of my productivity. That said, you don't have to buy into the culture either. I had 66 units in Fall 2017, which was tough. This semester, I'm at a nice 48. No one really cares. I still complain about my classes alongside everyone else.
From what I've seen of students at Heinz, those with the highest stress levels are those that came in with no/limited quantitative experience. Imagine trying to learn Python, R, Stata, and SQL at the same time while trying to grasp grad-level statistics/data analytics.