I actually have the same problem, started off as a mouse player and I'm left handed. I got a tablet a while back and decided to try it out. Aim is perfectly normal, but my tapping accuracy is completely off beat, especially on triplets. It's most probably because we're not used to tapping with right hand instead of left. ( I personally tap much faster with right than left.) But nevertheless, I still screw up quite easily even though I can stream faster. It just needs a sort of play style on how you tap to different hits. Like Alternating your fingers or something like that.