I've had no problems using the basic German infantry to hold the line. You take casualties in those units, but their base size is large enough to absorb them (120 men to 80 in a Roman unit on large). In defense stance, they will definitely hold long enough to flank phalanxes, particularly one you get silver chevrons on them. One key is to realize that those units will have to rotate back for retraining more often than phalanxes or Roman infantry would. Maintain a reserve to keep your best generals stocked.
My biggest problem is dealing with the Romans with their huge defense and javelin volleys. That tended to be bloody, but eventually number will win out. By the time Rome controls Italy and looks north towards Germania, you should control most of continental Europe. Macedon is the real problem, as you have to fight many full stacks to get to a port near enough to Greece to be safe for shipping an army into the Peloponnese.
I'll point out here that German units are far cheaper to build and maintain than civilized armies are. In upkeep, a typical high end unit like Principes or armored pikes will cost 250-350 per turn in upkeep. German basic infantry will cost you 113 and the elite spearmen will cost you 185. Sure, your economy starts small, but once you control Gaul and Germania, you can maintain a lot of stacks.
