Basically, there can be one or two politicians who run around saying "I'm A Different Kind of Democrat," but if more than that are doing it, if every ghoulish political consultant tells their candidates to do it, if the DCCC demands all their candidates do it, you basically have an entire election campaign of, "Democrats suck, vote for me, a Democrat."
Maybe it is great swing district messaging! If I'm the one guy who does it, +5 for me. But when two people do it, only +4. Three people +3. Every additional candidate who does it pulls down the vote by a point across the board. A dozen Democrats running against Democrats, and everybody loses. (Of course I am making up these numbers).
I'm not convinced it is good messaging, and certainly not across the board. Maybe if you're the genuine weirdo outsider who was an HVAC repairman until a month ago and are now running for Congress, you can do the Independent guy schtick with conviction. But "Hi I went to Harvard and Yale and spent 12 years in the CIA and 6 working for McKinsey, vote for me, a guy who is Not Like Other Democrats, for an independent voice in Congress" is not especially convincing!