In both Romania and Bulgaria (2015) I found I could cover only about 30 mph whether by bus or by train. Things may not be any faster in Sicily, but the distances tend to be far shorter, so you're not spending 1/2 day or longer for every leg of your trip. I have the impression that Romania may have a bit more rail service than Bulgaria, but given that the trains are about as slow as the buses, it probably doesn't matter.
Romania does have a couple of rural areas (Maramures in the NW and Bucovina in the NE) for which you really need a rental car or some sort of locally-arranged tour (the latter being pretty affordable). I suspect there are sights like that in Bulgaria, too.
One issue with the Romanian buses is that, as of 2015, many required reservations, but you couldn't take care of that at the bus station. You had to call and put your name on the list, then you paid the driver or a fellow employee right before departure. A very strange process and most inconvenient for tourists who don't have phones and/or don't speak Romanian. That said, both times I ran into that issue, a kind Romanian picked up her phone and made a reservation for me.
It's a very interesting area to visit. Costs are low, people work really hard, and many of the young folks speak good English, so you can have substantive conversations with them if you can catch them when they aren't too busy.