It's not actually the songs that make them similar, it's the standpoint of which they write from.
Jill and Adele both write from an emotional standpoint and find fuel from their feelings. This can also be said about Jazmine Sullivan.
Erykah, who I'm just throwing in to throw in, always wrote on the raw and spiritual parts of love. Jill was a woman hurt and alone, while Erykah is flying high after biting her man's head off. That's how I like to think of it.
Erykah vs. Jill: What Separates Them? is a good discussion.