In Team YankeeEdit
One of the few French vehicles that actually has a true AT weapon.
While AT23 is, of course, nothing to sneeze at, you'll undoubtedly have brought your fair share of French Milans (and probably a smattering of APILASs), which can kill most tanks just as well, so AT shouldn't be much of a problem. That said, certain especially heavy vehicles, such as the Abrams IPA1 and Merkava 2 (read: anything with frontal armour 19), are going to cause trouble even for your Milans. This is where that AT23 comes in very handy.
Mephistos have the obvious advantage of speed over infantry Milans, allowing for quick repositioning to adapt to unexpected enemy tank formations. However, the French have many vehicles that can mount those infantry Milans on their turrets, so even there the Mephisto hardly shines.
All of this makes the Mephisto a situational unit, really only justifiable if you are near certain of facing the heaviest MBTs in the game. Ironically, these are fielded by nations allied to France.