I know that this will come off super harsh, and maybe this is a Millennials expectations thing, but we LOR writers are under no obligation to be nice. We're obligated to be honest.
One thing about these letters, no matter what the venue, is that the judgment of the requestor comes into play. You should have a good relationship with your LOR writer. Why even take the risk of asking for a letter from someone who not write a good one?
This is why I always advise pre-meds to ask "Dr ___, do you know me well enough to write a good LOR for my app to med school?" So just change the wording for residency, and it's the same thing.
To be honest, I've told people "I don't know you well enough to write a letter" and also "I can't write you a good letter". To date, I wrote one bad letter, but that person simply didn't belong in med school.
There was grad student in my post-doc lab who was a real viper. She left before she finished her PhD, and got a technician job somewhere on the Eats Coast. One day, out of the blue, she called me and told me of some career plans and asked if I could write her a letter. "Sure", I said.
She never asked for the LOR, and too bad, because I would have put a stake through her heart.