AFAIK the conductors began appearing before Stagecoach took over, but as it is over 20 years ago I may be misremembering. Certainly, the system of buy before you board was doomed to failure from the start due to the unnecessarily complicated procedure adopted.
On the question of evasion, my observations are that this is not major problem. The vast majority of conductors are very on the ball and will approach everyone who boards, except on the rare occasions when extra heavy loadings mean they cannot get through quickly. Of course, a few conductors are less conscientious, but anyone from Rotherham used to not paying when going on the train and seeking to regularly evade fares on the tram/train will be in for a rude awakening.