I think the earlier stage you are (seed, angel) then yes, there is definitely risk for a large class of employees. You're taking a risk on the founder's ability to execute, joining a company that is not cash flow positive and tying your mortgage payment/insurance to their ability to raise money in the future... it's derisked in each successive round and startups/founders are not all equal, but I think if we're talking 1rst employee it's fair to say there's some risk.
Yep...if I'm going to be offered $x as employee #3 with little to no equity, and the same $x at a stable company...I don't see why I should risk joining when there's little reward.
Risks can be:
Stress due to working more (wearing more hats, not enough employees)
Not having a stable paycheck because company needs to pay venders otherwise they go bankrupt.
Recession or downturn, loss of job immediately while big company can weather.