Roman Republic Accomplishments

[Kraxis]

Actually the Empire continued to believe that one day it would conquer Germania. Tacitus gives a picture of a military that more less says "yeah, we have tried for 200 years but damn you we will conquer them eventually". So it wasn't as if the Romans just hunkered down on the Rhine-Danube line awaiting the Great Migration.
The simple fact was that the Empire COULDN'T win in Germania.

After the initial conquest, which was actually going pretty well until a Roman citizen chose to rebel, the Romans only managed to raid, ranging from very good with Germanicus to downright lousy. But despite buying off the Germans continually (and thus making them richer and stronger in time) the operational tactic was to conquer. The Empire just wasn't able to.

In the East Parthia was all that strong really. If the army failed the social structure would be shaken. But again, aside from a couple of serious effort later on there was not done much. Not because it wasn't worth it (S. Severus showed that and he only plundered), but because the military means weren't there.

The conquest of Britannia wasn't made because of economic grounds. Britannia was not worth it really, and it was only through luck that the locals proved to be adaptable and in the end got their province to be pretty prosperous (but never really rich). And it wasn't conquered by two legions but four. The II, IX, XIV and XX legions all served there and at the same time, along with about 20,000 auxiliaries. Sure this was after the Triumph in Rome but you can't say Britannia was conquered when Claudius claimed victory. Caratacus kept the Romans guessing and it took time before victory was in hand.
Hardly a sweeping victory.

The Roman Empire just wasn't geared for conquest like the Republic was. The incentive for conquest had vanished as soon as the soldiers were not citizens (you know what I mean).
In the Republic the soldiers got the spoils of war, this meant that they would sign up for war if it looked to be profitable (or if their homes were in danger of course). You can see this by the amount of loot people like Cato (the Elder) and Tiberius Sempronius Gracchus brought home from Sardinia and Hispania. Or for that matter the war with Antiochus III.
They were the wars that made Rome rich... filthy rich in fact.
These armies were comparably cheap (not cheap per se, but much cheaper).

The professional armies of the Empire were expensive, and thus the will for staying in Germania evaporated, while the citizen army had endured centuries of hardship in Hispania, which aside from the initial conquests weren't that wealthy. A similar situation could be imagined in Germania had the armies been of the citizen kind.

The Empire couldn't keep it up. The losses would drain her, which they wouldn't before (2nd Punic and the Teuton War were far more expensive in terms of manpower).

[Atilius]

The conquest of Britannia wasn't made because of economic grounds. Britannia was not worth it really, ...

Yes, this was a prestige-building exercise undertaken by Claudius just two years into his rule.

And it wasn't conquered by two legions but four.

Kraxis is correct here, I misread my source.

They were the wars that made Rome rich... filthy rich in fact.

For instance, the tax on Roman citizens' property (the tributum), had historically provided most of the Roman state's revenues. Upon the conquest of Macedonia in 147 BC, it was abolished and never reimposed.