Lots of people do those drives. The Arenal area is very spread out, so a lot of people like to have a car there. We just use private drivers and then take taxis once we're in town, but they do add up. Things in MA are a lot closer together, so you don't really need a car there. Where we stayed we were able to walk to several restaurants from our hotel, and for those further away taxis were pretty cheap. You just have to be careful walking along the road. I'd say it's pretty safe as long as you don't drive between towns at night and you never leave anything in your car unattended. I'd look at using a GPS if possible since things aren't always clearly marked. You can encounter a lot of rain and fog on the way to Arenal, but less likely in March.

Rent a car in CR is the best way to travel it, hopefully with a GPS if this is gonna be your first time in CR.
It is not dangerous, the only recomendation, dont drive when it is dark...