Those rebels had crappy levy and slingers, and Suebi have 9 regions by now with full control over Hercynia and Magna Germania, and had about 5 armies (Surely they have more, I just don't see those) with berserkers, onagers, archers.. Well, they had alot of high-tier units.
But thats not only about Suebi. In all that time which I've spent in this game (And that's 400+ hours by now) I've seen only few times when AI attacked rebels (The actual, non-faction rebels). Sometimes it rebellion right in AI's original capital, and then AI doesn't even care that their capital is in hands of rebels.