They are really the same. I think often when we use “I’ll level with you” we are about to say something that is a bit shocking or exciting to find out.
If I say “let me level with you”, I might be meaning “let me make this very clear so there is no misunderstanding.
If I say “let me be honest with you” I might be going to say something that is hard for the other person to hear, but I am trying to be as kind and sincere as possible.
Hope that helps. “Let me level” with you is not rude, it is maybe just more intense.