There's quite a difference between barbarian and Roman naval units on Total War: Rome 2.
In the video below, The Creative Assembly's Bickham shows off how the heavy barbarian ships aren't as well-equipped for ramming as the mediterranean equivalents, but they are very heavy, and can take a lot of damage. Once the ships engage, the differences between the units onboard makes more of a difference, although slightly more distant vessels loaded with archers have an impact. On top of all that, you've got to think about whether smaller ships can outflank the heavier ones, the way cavalry rings infantry.
Total War: Rome 2 arrives on PC on September 3.