By joining a late stage (vs early stage) startup as an employee, you are trading execution risk for valuation risk. At an early stage startup, your shares are

By joining a late stage (vs early stage) startup as an employee, you are trading execution risk for valuation risk. At an early stage startup, your shares are essentially free to purchase - especially if you join a company which hasn't had a formal external valuation event (like a fundraise) yet. All your risk is around the startup evolving into a successful business with a high value. Join a late-stage startup, and most of the execution risk is gone. But the company may already have an artificially high value attached - so you have a lot of risk that your shares end up being worth significantly less than you paid for them. The real unicorn, for an employee, is a middle- to late-stage company which has successfully executed, is growing, and ideally hasn't had any formal external valuation events. There your shares are cheap to buy and extremely likely to increase in value.