Pros
Will totally consume you. Surrounded by other workaholic, brilliant people, that energize while really the needle for sustainability. No limits. Can take on as much as you are capable, but they will hold you accountable. Extremely well funded and compensation/equity available. Principle centered leadership and values. Results, results, results. You can argue anything so long as you bring facts/data/etc. No-one keeps track of hours, since people work 24X7. CEO is a successful serial entrepreneur and venture capitalist.
Kontras
It is a young company and lines of responsibility, authority are blurred. You can't wait for someone to tell you what to do or what not to do. You have to learn then perform in real time. People move fast and there isn't time to give problems/solutions time for careful consideration. Very, very high bar standards. They expect as average, everyone to deliver exceptational. Opinions without data, backup is not well received. Don't bring sit on a problem, but don't bring up a problem without a thoughtful/effective potential fix.