It was the voice direction that nabbed the last two seasons. There were many talented VAs who's performances really suffered as a result of bad voice direction and rushed production scheduling. According to Linda Ballantyne, they basically recorded each line twice, and went with the better one!
Though I do have a problem with Vince Corazza's "My Love!" in the R movie towards the end. It was one of the worst delivered lines in the whole movie. Though Vince was usually awesome, so I can forgive him!
As for WHY Emile Barlow voiced both Mars and Venus at different points, Katie Griffin (the main voice of Mars) was away shooting a movie, during production of the last 17 episodes of R (which were produced after a long hiatus), and Emile Barlow filled in while Griffin was away. When production resumed on the three films, Katie Griffin returned to reprise her role. When production came back from an even longer hiatus for S and SuperS, Optimum Productions was contacted by Stephanie Morgenstern (the voice of Sailor Venus)'s agents, who said that she had moved on to more on-camera acting, was no longer participating in voice acting, and would turn down the opportunity to reprise her role. They subsequently held open auditions, and Emile Barlow successfully auditioned and became Morgenstern's permanent replacement for Sailor Venus. She was obviously no-longer voicing Sailor Mars at that point (Katie Griffin reprised that role for both seasons), so it wouldn't seem as noticeable.