Pros
Engineers' working environment is great. All engineers are working hard, smart and kind. Working hour is very flexible (WFH, come late and go back late OR come early and go back early).
This company is real start-up and you have to do everything as an engineer. it's good thing to develop your career.
Kontras
Working hour is flexible but typically long. You may have to work after going back home, and weekend(not always though). Engineers often have to do things that are outside of the job description. You sometimes have to fix requirements because there is no one else to do, sometimes have to manage schedule and organize things with people in other teams because.. and so on. It's tough to just focus on writing code.
Also, in these days the company's business doesn't run well, so you may feel you job insecure(the company has performed lay-offs several times). But that's not quite different as other start-ups.