Well, after being in the hospital a few years I got frustrated with and tired of the hierarchy, the rules, the pace, the exhaustion, the lack of appreciation, and not feeling like I was making a difference. That's why I wanted to switch to public health (where I may not feel like I'm making a difference either!). I liked ER but got burned out, I suppose. And global health has always been my main interest; disparities in health that are related to socioeconomic conditions, etc. I taught nurses in Malawi and Afghanistan, which was challenging but also rewarding in its own way. More my style, I think, than one-on-one bedside care.
If you're smart and committed (which you must be!) you'll do fine at UCSF. It's rigorous but not more so than other schools, I suspect. I was challenged a lot, and stressed a lot, but it's doable. Sadly the program was lacking in the global health area, which was a real disappointment to me. In fact, they're changing the name of the specialty (so I hear) from ACHIN to Advanced Public Health Nursing. So if you're hoping for international perspectives and opportunities, there are slim pickings! But in the end you'll have those letters after your name, and that's what will open doors. Or so I hope!