I believe the answer is:
- Advantage of The Britons is that they had already gone into the iron era which Rome was just introduced to
- A disadvantage for the Britons is that they are strategically inferior compared to Romans.
Being the first to get into the iron Era means that the Britons have implemented better materials into their equipment, shich give them military advantage over the Romans.
But, Julius Ceaser was regarded as a strategic genius across European continent, so he manage to outsmart the Britons regardless of their advantage.