I've worked for a startup similar to that, though it was series A when I joined. It was gross. I stayed there for nearly a year and a half because I did genuinely enjoy the work and my peers, but ultimately the founders ruined the experience for me with their evasiveness and lies, and their creepy later-on focus on "loyalty" when the company's prospects started to go downhill. The last straw was when I was told by another often-in-the-know employee (whom I trusted) that one of the founders had found out I was interviewing at another company, and he called someone he knew there and told them not to hire me. Obviously I don't know for a fact that's what happened, but it sounded believable based on the founder's other behavior, coupled with the seemingly-fantastic interview experience I had.

That hurt, but I realized I had to do a better job of treating interviews as a two-way street. The company was interviewing me, sure, but I also needed to interview them, and learn what I could about the kind of people the founders and my peers were. I also needed to understand up-front what would be expected of me, and how flexible they could be with my time. The next startups I worked for were much better, and I never felt exploited.