Down and in is much more effective for most RHP against a righty than against a lefty. Usually a RHP fastball tails into a righty and if it is in enough that helps the pitch. Against a lefty that movement brings the ball out toward the head of the bat. The exception is someone like Scott Shields who has so much movement that he can get called strikes with the low and in fastball vs. lefties. The problem with low and in, RHP vs righty, is that there are righties who will simply drop the head of the bat on the ball and not get jammed and then the ball goes a long way. There are some righties who tend to swing around the ball a bit and they usually have a hole in their swing down and in. The key is to be able to recognize the difference between the hitters.
My experience as a RHP was that if you can keep the ball in on the hands it is better to be at the waist or above vs righties although if you miss over the plate the ball will go even further. Similarly, my slider was more effective against lefties when I got it in on their hands waist high than it was low and in although both worked reasonably well. Against righties I mostly kept the fastball low and away but had to mix that up with a low and away slider and an occasional fastball up and in for it to be really effective. Against lefties I threw the fastball almost exclusively low and away if I could hit my spot.
Even today I have problems throwing BP to lefties because I automatically throw it low and away to them and it isn't good for their confidence.