The Roman expansion seemed to have stopped whenever they ran into cavalry-heavy enemies.
The Romans conquered the Persians at one point. Couldn't make it stick for more than a generation, but they managed.
They also conquered Spain and France. Against those barbarians that you think beat them.
The Romans did NOT successfully defeat the Germans, of course. Teutoberger Wald wasn't actually a case of "asymmetric warfare", since it involved a large Roman Army and a larger German Army (and a stupid Roman General, which was the real cause of the Roman defeat).
Of course the Romans never made a serious effort to conquer Germany. Why bother? The place was a worthless hellhole. Even Big Julie (who led punitive expeditions into Germany a couple of times) didn't think it was worth conquering. And the later Roman generals who led punitive expeditions into Germany whenever the Germans bothered the Romans living in France at the time didn't think Germany was worth bothering to conquer.
Not that they ventured often into those territories, but if I recall correctly, whenever they did, it was without significant success.
You don't recall correctly.