This behavior is extremely damaging to the startup scene. Who would join a startup these days unless it’s run by a close friend or relative? At least in that case, the scorned junior employees would have social recourse.
If you join a startup, and have equity that isn’t special in some way (defending against liquidation preference or dilution), you’re the sucker. You’re just going to grind for someone else’s payday when a deal is made in a room you’re not in. You’ll only be made rich if someone with the power to drive the decision thinks you should be. As always, it’s who you know and being likable.
If you get an offer from a startup you'll need to pay a great lawyer to negotiate an iron-clad contract, so much so that any post-1st round startup would rescind the offer.