I have an Extreme on the front of my LHT. It is rock solid and I like the platform. I put Ortlieb panniers on it and there's no attachment problem. I have cantilever brakes - no problems. The only problems was the spacing didn't match the spacing of the fork bosses. I had some spacers from the Tubus rack I was replacing. I used those and they fit perfectly. If I hadn't had those I would have tried washer, making my own spacers out of something at the hardware store, etc.

My wife has had a Jandd rack on the back of her Rockhopper for at least 10 years. It has also been rock solid - no problems. I installed it so long ago my memory is fuzzy, but I don't remember any installation troubles. All she has used it for is a trunk bag - no panniers yet. (One day I hope she'll have the time and inclination to try a tour with me.)