I’ll be honest with you.
If I am being honest. Which I am.
There are few things about communication that I find difficult to read. When we are reading, we expect our content to be factual and true, with few exceptions. Writing or saying “I’ll be honest with you” is completely unnecessary. It takes the value of writing down substantially. The phrase can lead readers to suggest that you might not be producing clear words otherwise. No one should have to preface a story with a caveat of honesty.
What if someone replaced “I’ll be honest with you” with “If I am lying to you”, then told you something. You know the something is going to be absolute nonsense. The content has a value of zero.
So unless you are writing a work of fiction, or quoting someone directly, there is no need to assure readers that you are being honest with them. We all are hoping for that in the first place.
Otherwise we would not be reading the content to begin with.