Darnellius,
I have tried ribs, butts, and briskets with mustard, just rub and oil (EVOO). I have the follow observations to share. These are from my experiences on my electric smokers (TEFS and TEFS, Jr.)
Rub only...this creates a dryer meat and the bark build-up seems minimal...it also does not attract the smoke like having an oil-base for it to adhere too?
Mustard with rub...This works quite well as a base for rub and smoke adherence, there's little to no mustard taste afterwards and the bark build-up seems adequate? Mustard is used as an emulsifier for mixing oil/water in other settings, so I would assume this is the reason why it's popular?
Oil with rub...for me, this seems to be the most consistent for smoke flavor, bark-build-up and appearance, the meat gets that nice dark brown/black build-up and the the oil attracts the maximum amount of smoke flavor. I cannot exactly say why this is...but I will assume it has something to do with the electrical charge of oil vs/ water? Water is a polar solvent, meaning it has a slightly + charge from the oxygen binding, where as oil is a non-polar solvent?!
My $.02 worth!
Happy Smoking!
~Brett