Hey, it's Tuareg!
Teutoberg wasn't successful because it was an ambush in the forest. The Romans were perfectly capable of fighting their way out of forest ambushes, as they almost did at Teutoburg. The thing with Teutoburg was that it had just rained-the ground was a mess, and the Romans couldn't find any solid ground. THAT was what did the Romans in. It is really an insult to the Roman fighting abilities to say that just ambushing them in a forest was enough to defeat them. Remember, Teutoburg took 3 days-and in a similar style ambush Ariminius tried on Germanicus, this time on solid ground, the ambush backfired pretty badly and the Romans easily fought themselves out of it.
So just saying, "Well, the Romans are gonna get ambushed in forests" is, quite frankly, a ridiculous argument.
I explained in this thread and many other threads why economical viability is far from the only concern (many times not even the main concern) for Roman territorial expansion.
And I swear if anyone brings up Hadrian...Mesopotamia in 117-118- a province that Trajan had already effectively withdrawn from, was almost completely out of Roman control anyway by the time it was abandoned, and had just finished being conquered (all the while there's a Jewish revolt going on, and Hadrian has to get back to Rome quickly to secure his position) is a completely different situation.
Ambushes in forests or elsewhere are militarily dangerous. Germanicus' troops were steadier than those of Quinctilius Varus. Even so, if my memory of Tacitus is correct, they needed to be rallied after almost panicking. The psychological effect of the clades variana was not inconsiderable.
I have only just joined this forum and so have not read your other threads. Nonetheless, economic viability was an important practical consideration for the reasons I have given a few posts ago. Drummond and Nelson, The Western Frontiers of Imperial Rome, has some good work on this.
It is true that Britannia was an economic liability when Claudius conquered it in the mid first century, but it had potential. By the fourth century, it had become a net exporter of grain to the Rhenish legions. But it took considerable investment to achieve this and the N and W of the province were always poorer and a net liability. And yes, Claudius' motives were principally political and military - the need to acquire Gloria because of his rather unorthodox accession and to prove himself to the army - but Tacitus in the Agricola also mentions the desire for British pearls as a motivation. E. Germany and Poland were simply impractical as sites for Roman provinces.