Going in they have a lower salary than if they work for a more established company.
Then, either their shares end up being worthless, or at the final exit, they make less money than if they worked for a more established company the entire time. IE: Being an early employee in a startup is a lose-lose situation.
This is something founders need to understand when recruit their early employees: These are often the most critical hires for the business, and therefore it needs a high probability of upside.
IMO: A series of retention bonuses, and/or guaranteed bonuses at acquisition / funding events is a good solution. It's how I've sidestepped the equity issue when I was employed during an exit event.