It seems to me that it comes down to several points.
1. Even before the Arverni-Aedui wars there were conflicts with the Romans. Most of these battles were won by the Romans. Therefore Romans>Celts most of the time. Yes yes, Brennis and such. Romans still won most of the battles prior to Marius and especially after Marius.

2. I dont buy into the idea that the Celtic tribes were weakened. If you look at the battles and numbers during Caesars expansion into Gaul, the numbers do not bare this out. In the battle of Alesia there were over 300,000 Gauls, not to mention 5,000 of them were the elite Arverni Gaurd. I know there was a conglomeration of tribes, but it has almost always been that way. Soldiers die, new ones are born and trained, this never stopped happening in Gaul. The Gauls were a tough and warlike people(I know they were also other things).

3. The Teutons,Cimbri and Ambrones (TCA)were certainly not vassals of the Boii. Yes they were repulsed by the Boii in hungry. I do have questions about this particular situation. Every thing I read on this (which isnt much) leads me to believe that is wasnt so much a victory as it was not worth the effort on the Germans part. I keep reading repulsed as opposed to defeat in the books I have read but none tend to go into details on the situation. I know the Boii were a very tough tribe so I dont have a problem believing they out fought the Germans. But the way its been presented in these books (again not much detail of the battle(s) ) it lead me to believe the Boii were in a strong defensive position. It couldnt have been much of a battle as the (TCA) kept on moving without much interference. If anyone can give me any information (books, websites) on this, I would like to see it.

4. The TCA entered Gaul and began rampaging through there which caught the attention of the Romans who's allies the TCA were attacking. The Romans came up and got hammered by the TCA and the TCA kept on pillaging. The TCA split up and some went into spain and pillaged there until they were repulsed by the Celt/Iberians. Again the TCA must have come out without much losses because they started again through Gaul and besting the very tough Belgae.

5. What Im trying to get at is that the Stats in the game for the Celts are rated to high for their units. The Celts were most certainly a tough group of people but they were bested by the Romans and the Germans Most of the time. And I still dont buy into the "they were weakened" argument, the numbers just dont bare it out. The Gauls still had lots of well trained cavalry and elite units, even in Caesars time.