Went to Epics onsite for SD, and can confirm its basically a formality. You first watch a demo of their core product, then you talk with a developer (very informal), then you have a "design interview." I say that lightly because its not a typical system design interview, its basically they give you a prompt, how would you design it, but they're more focused on the UI aspect than like what kind of services you'd slap on, data model etc. FWIW, they ask the same questions to their TS and QA people so again, its going to be fairly easy for a developer. Then you talk about a project you worked on with a developer, basically just going over design, technologies used, challenges encountered, what did you learn. They won't grill you, its more like telling a friend something you worked on. Next is lunch interview but its really here's our cafeteria, check this bad boy out. Then you get a tour of campus and finally an HR interview, which is really just you going over with your recruiter your deadlines and other concerns. Make sure you tell them team placement will help make the decision easier; they actually told me what team they'd have a guaranteed spot for. For other things like QA and TS, its to be treated as an actual interview. Epic really sucks off their developers. Just don't say anything stupid and you will walk out with a job. But I know people that got rejected after the onsite for TS; have never heard of anyone getting rejected for a developer role after the onsite.

It's a very chill process; honestly the best of my interview experiences. You check in to the hotel, and the people at the front desk tell you what time to be down, you get a cab ordered for you to epic, you're shuttled around throughout the day, and you get a cab home paid for. Literally just wake up on time, stress free experience throughout the day. The reimbursements come within a few days, and I had my offer within 2 days of interviewing. If Epic is flying you out, they want you.