Power imbalance doesn’t have to mean who has a job, who has magic, who has friends, ect. It’s about respect and consequences of actions.
If Rumple or anyone else–Regina, Hook, or even someone we consider “hero” status from the start–can make morally black decisions and not face consequences of those actions from their loved ones then there is a definite power imbalance. Staying with the Rumbelle example for a moment: Rumple feels that he can have his cake and eat it too. He can get revenge on Zelena through murder but also lie to Belle and then get his happy ending by marrying her. He doesn’t stop to consider that this could cost him everything. And when there is a chance that he is discovered in 321, he simply magicks it away. Even after Belle learned about Milah in The Outsider, she did not leave. She took the news with a straight face and then said she’s never stop fighting for Rumple. In other words, she has basically given him permission to do as he likes because she hasn’t the heart to stomach to leave. He is the power in the relationship because Belle won’t make him face the consequences of his actions. The one time she did was the Crocodile when she walked away, but she came back and then stayed even through a bunch load of other stuff including: beating Keith, not helping to save SB in 222, ect. That’s a definite power imbalance.
Cora and Henry Sr have the same thing: Cora can do as she likes knowing that Henry Sr will not stop her, including being abusive to his daughter.