Working at an early stage startup provides tremendous opportunity for a person to work not only in their particular area of expertise, but also learn new skills and contribute to new areas. A small team means it is likely incumbent upon the existing employees to figure out solutions to everyday problems rather than bringing in additional people and/or resources, at least in the earliest days of a company. This reality rewards those who are inquisitive, have initiative, and are able to learn quickly. That is all to say that any particular day is likely to be different than the previous or next days.
In my case, I have a legal background and am involved in worldwide expansion of our product at a medical device startup. A day may involve working on new license agreements, managing our intellectual property portfolio, or interfacing with our end users. Additionally, I work on a lot of documentation related to implementing our software in new locations. This involves writing and learning about new technical areas that may be outside of my previous experience.
In summary, working at a technology startup provides opportunity to grow your areas of competency. The very nature of a startup requires everyone to be striving for the same goal and working effectively together. It provides learning opportunities that a larger organization likely cannot. It definitely requires some risk, but the benefits outweigh those risks in my mind.
I would recommend continuing to ask questions on this platform, as well as talk to any friends you have who may work at a startup.
Best of luck.