I've never gotten a letter for anything I've downloaded via private trackers. On the other hand though, I use it relatively infrequently compared to anime I get from public trackers.. and I've only gotten one or two of them from FUNI and others.

Statistically speaking, the lack of letter from private trackers would be insignificant when you look at it like that, but you can argue that the differing materials are not protected under equal aggression.