So whether or not Endora was right about him is irrelevant. Nothing Endora did to Durwood ever forced Samantha to abandon him. Neither did his own sexism and domineering nature. The real question is how dumb a blonde is Sam for being so powerful and jumping headfirst into this situation willingly anyway?
The idea, as I understand it, was that Samantha was drawn to the mortal life, an existence where you couldn't get anything you wanted just by wiggling your nose but had to earn your accomplishments. So yes, she was powerful, but she felt that power, used too casually and carelessly, had left her culture decadent. Think of it this way: if a spoiled rich girl falls in love with a working-class wage slave, and she gives up her easy life where everything is handed to her, marries him, and has to toil in the trenches to earn her keep alongside him, does that make her dumb? Or does it make her braver and stronger than the rest of her family?