It's not that hard but you definitely have a really hard time with amphitheaters without a car, public transport to those doesn't really exist. Getting a cab back might be a problem with all the after show traffic, and is going to be expensive if you are stuck in traffic. Arenas are usually directly in the major city centers, so that is a plus. Amphitheaters are not in cities, they are in tiny little shitty towns, and not even in walking distance from there.