Pros
The coworkers are fantastic. I can count on people I work with to be smart and good at their jobs and to help me when I need it. I feel like what I work on makes a real difference in people's lives, helping doctors and nurses take better care of patients. Overall, the company is ethical in its dealings with customers and generous with charities. The compensation is generous. Upper management communicates at length with employees in monthly staff meetings, so you know more about what's going on than you would at many companies. It is privately owned, so you can be confident things won't change abruptly in unpredictable ways due to acquisition, merger, or generally stockholder pressure. Epic is a process based company, and the processes have logic behind them.
Cons
The form opinions about people quickly, and it can be hard to change them. The flat hierarchy is good for many things, but "advancement" in terms of higher titles and more official authority are frequently unavailable. If you get a less experienced manager (which can easily happen), you need to help him or her manage you or you can end up with too much work, and I see plenty of younger employees who struggle with this. Internal development management tools are awkward, and improvement of them isn't a high enough priority.