For the Celts they succeeded in conquering (not the Irish/Scots or the Picts) with hundreds of thousands slaughtered and literally millions enslaved, the price of that progress was very high. Were they alive today one could ask Vercingetorix and Boudica what they thought of Roman civilization.
Bahh!!!
The Romans incorporated the Celts into their political and military system.
Even the much maligned Julius Caesar recruited an entire legion, the Legio V Alaudae “The Larks”, entirely from the Gauls. The Romans were conquerors - they beat up people who opposed them and welcomed and incorporated those who did not or who were conquered and survived. It was part of the strength of the Empire - making the conquered into Romans and then incorpating them to spread the Empire. This is very unlike the Assyrians, Babylonians, Persians, Greeks, etc. who never really made any attempt to assimilate conquered people into their political, economic, social and military systems and share power with them.
Vercingetorix lived out his days near Rome, on a pension, basically under house arrest. Boudica was the Celtic leader who mass murdered thousands, so her complaints would be hollow.