Having driven both LHD and RHD on both sides of the road (as required by the country I was in, not just for fun!), I would say that there is definitely a significant advantage to being on the “correct” side of the vehicle in terms of day-to-day driving, but as long as you’re not stupid it’s not difficult to make the change over when you change countries. It’s easier to flip your brain to the other side of the road if you’re in the “correct” vehicle for that country, because everything is just reversed (inside and outside the vehicle), but it’s not hard to do in your normal vehicle either. I’m trying to think back… I can remember pulling out of a side road onto the wrong side of the road one time in Cuba, but other than that I’ve never had a problem adjusting.
Oh, and roundabouts take a little thought for the first one or two after changing over, too. But that’s it.