"frankly disagree" is correct and can be used in written English.
You can use it when you want to strongly express that you do not agree with something. Example: "I frankly disagree with the decision to cut the budget for our school district."
"That's the problem with Washington, quite frankly — if you disagree with somebody, you should dislike them, too," said Hagel, who is sixty-two and has white hair, a worn, ruggedly handsome face, and a forthright manner.
He put together a far more diverse (not just a CIA/military) team of advisers, deliberately forced options to be aired and debated, accepted no easy assurances of military or diplomatic success, and created a decision making atmosphere that invited people to speak frankly to, and disagree with, him.
She said that special friends "can talk frankly and we can disagree from time to time, even sometimes fall out over a particular issue," but that disputes "can be quickly overcome and forgiven".
I agonized over whether to quote "A Midsummer Night's Dream" or "Romeo and Juliet" on the response card — and then I agonized over whether to use a semicolon or a comma in said quote, because frankly, many Shakespeare authorities disagree on that.
I completely understand and frankly I do not disagree.
These guys are seen as heroes to many Thais, and frankly I can't disagree with them.
(p. 177) I disagree and frankly don't see how anyone who works closely with any of a wide array of animals could remain skeptical and agnostic about whether they are conscious.
Senator Jeff Merkley, Democrat of Oregon, disagreed: "Frankly, your response frightens me because I think there are occasions that they're in conflict".
He said: "We're familiar, too, with their frankly patronising approach to those who may disagree.
