However, being an employee in an early stage startup is a great way to jumpstart your career, whether you have entrepreneurial aspirations or not. In an early stage startup you are going to be rubbing noses with investors, working on interesting/tough problems with very smart people, taking on huge amounts of responsibility, getting in way over your head every day, and generally beefing up your resume.
Early stage engineers do not usually work on interesting problems at a startup. You are hired to do the uninteresting stuff. A startup may or may not beef up your resume. It's still easier to get hired if you have Apple or Google or Facebook on your resume than a failed startup. You generally do not rub noses with investors, but you might be invited to an informal lunch if the company is still very small. You do take on a lot of responsibility and get in over your head. It's a good jumpstart to a career if you can't get a job at Google or Facebook or Palantir or Apple or Goldman Sachs.