It generally goes like this:
The founders who promise lots of equity by joining early are usually unintentional scam artists. They offer the world, but these founders are taking a stab in the dark (even though their idea might be good and well-intentioned) and generally have no real plan for execution. They're usually great salespeople who help you buy into the vision, but since they don't have a plan or the connections they need to make the company successful, you should stay away at all costs.