With all-metal wheels you can get away with a bit of sliding, but having traction tyres, on one side!, made life very difficult. Personally, I hate rubber on the rail, and finally persuaded him to put four all-metal wheels on one of the power bogies - it's been OK since then.
I tried it with all eight powered wheels un-tyred, but we have a 1/35 slope whilst the end of the train is still on a bend and it wouldn't have it - maybe with more adhesion weight we could have improved things, but he's happy as it is now...