His acts all have good things to say about him, which I think says something. Even when he's said he picked the wrong song, none of them have complained.
I don't think you can judge on what kind of a mentor he is, as it's all behind-the-scenes. However, if you're basing it on something, how about jogging/training with Craig, and sending a nutritionist to give him a customised diet? [And it was Craig who mentioned wanting to lose weight.]
Frankie was a shit, but we don't know what happened. Could be he was told to take him on. Look at Kelly choosing Janet over Jade. That was an awful decision, but doesn't make her a bad judge/mentor.
I don't like him because he's a Tory

but I think he's a great judge. It's very refreshing to have judges who actually know what they're talking about. Hearing Cheryl critique singing was like an awful joke, especially with Danyl.