Only thing I can really fault Mythic's interview process for is the single most important thing: the end result. Their loss. I know full and well I can do the job, and I would have stayed to the end if given the chance.
It starts with a phone screen, followed by an on-site. The center of the on-site is a 2-hour technical challenge, which is tailored to your particular role you are interviewing for. They must spend a substantial amount of time on this (I anticipate Mythic might start getting "interviewing fatigue" before long. So this might change later). Be prepared with loads of coffee and energy drinks. and bring your A-game. And don't try to BS your way through. Frankly, I think these challenges are designed so that--at most--only the very tip-top candidate can complete it (if that). They give you a very short time window to complete it, and really they are more interested in seeing how you behave under the circumstances. I can't honestly fault them for that--that's exactly the way I would have done it.
HR was very up-front about everything. Which is refreshing, in a professional world where recruiters' communication normally is pretty dirt-poor; especially when you don't get the job. I liked the office, I liked the people. The work is meaningful. Not getting the offer was downright heartbreaking. But thanks to HR, I know exactly what it was: it had more to do with my fit for that particular role, and not about Mythic.
Overall, I am probably a better person having done the interview; offer or no offer. However, I would have really preferred not to have spent all day + time prepping on it, if my chances for that particular role were slim to begin with. However, nobody's perfect, and I liked Mythic as a company.