90% of the startups fail and a lot of them fail before they make profit or even are able to pay a reasonable amount to the people who work for them.
However, for those who do have the option to take a financial risk and / or have an entrepreneurial way of thinking, for those who love the drive of people working at a startup, the upside can be big. Some startups become good running businesses and some, a very small amount of the remaining 10% will become very big.
Having joined a startup from the beginning and it becomes big, very big, the value per employee can skyrocket. And a large part of those companies do recognise the endless efforts the employees have put into it to make this happen.
As you can see in the above bubble chart, some companies have a very high value per employee. So, yes, working for a startup can be a risk. However, if it becomes a success it can become a very big success with a big financial gain. You should however never join a startup with this as a goal, join a startup for the fun and for the mindset that lives in such a company.