The Romans were always willing to go along with the beliefs of conquered peoples to get along. They usually adopted or otherwise cooopted the gods of those peoples, often as a weapon of war (there’s a term for that, but I can’t recall it).

The druids had two big things against them. The first was their practice of human sacrifice. There were plenty of lurid stories about using vicitms as messengers to the gods or the dead, wicker men, etc. They’re all there in Caesar.

The other problem was the political role the druids played. The druids were a professional priestly class, unlike the norm in the Mediterranean world where priests were usually chosen from among the citizenry. They held a lot of influence on the local chieftains, who had to work hard themselves to get their men to follow them. One could say that the Celts allowed themselves to be led rather than following anyone. It’s a lot easier to get people to do what you want when you can cause them all sorts of trouble with the gods. The druids were the one thing that had any kind of federating influence on Celts (nothing could have had a unifying influence). If I remember correctly, druidic influence was even one of the excuses that Caesar offered for his attemped invasion of Britain.