Friction shifting will always work if assembled correctly. If levers are slipping it is because there is an error in assembly. Period!

I personally despise the Simplex retrofriction shifters as they require what I feel is an inordinate degree of motion to shift. When fitted with a Mavic SSC derailleur with a 6 block, the arc of motion that was required exceeded 135°. Both types of Campagnolo retrofriction levers I tried worked in less than 100°, in keeping with what I have on all of my other bikes. I suppose it is all a matter of getting used to the action.