Our trip a year and a half ago (at Eastertime) involved a twice-delayed ferry because of rough waves to Hydra, and then a shared taxi to another port for a less turbulent water crossing. So some flexibility and an extra taxi fare got us to Hydra.
After crossing from Hydra to the Peloponnese, we picked up our rental car. Driving wasn’t technically more difficult than in France, Italy, Slovenia, Croatia, Bulgaria, Spain, or Portugal. And driving on the right, as in the USA, it was easier for us than in Ireland and Scotland. The one Greek issue was so many people cutting across corners, even on blind curves, and heading at us in our lane. Everyone moved back onto their side of the road in time, but it was disconcerting, time after time. Just anticipate it, and leave extra room. Oh, also some signs were in Greek only, which took extra time for us non-Greek-readers to try to figure out, including names of towns.
We didn’t venture north of Delphi, and maybe things are different farther north? Compared to some parts of the world, though, there seemed to be less speeding or tailgating.