I wouldn't say I'm an expert on the Germanic tribes, but having read fairly extensively on Caesar, Varus and Germanicus, I can give you some background.

The Romans actually did a pretty good job of whipping German butt. At the end of the first century BCE, Tiberius (soon to be emperor) and his legates had done a damn fine job extending Roman borders East of the Rhine and North of the Danube rivers. So much so, they declared the tribes there pacified. Tiberius peaced out to go and deal with an Illyran revolt and Varus, a former governor of Africa and Syria was appointed to watch over these lands with three legions (soon to be extremely infamous legions).

Enter Arminius, a commander of auxiliary German forces during the roman subjugation, a roman citizen AND a Germanic Prince of the recently subdued Cherusi? tribe. He was a sneaky fucker. Playing off Varus' lust for political glory and his own ambition to unite the German tribes, Arminius convinced Varus to march north with his legions to proactively subdue a rebellion (which he was organising).

Varus being impetuous and also extremely trusting of Arminius (they had been bros for ages), rallied his troops. The legions were the 17th, 18th and 19th and took off toward modern day Mainz to expand the borders further and expand Varus' political penis.

What followed was the greatest ass kicking the Romans would receive since Cannae... The Battle of the Teutoburg Forest.

Led down a narrow trail, which according to Cassius Dio, had been ravaged by storms, the Romans were ambushed. Unable to properly form up and completely cut off from a retreat, the three legions were harassed and slaughtered all day and night as they made a cannonball run down the trail in a doomed attempt to punch free of the hell they found themselves in. I get chills thinking about how horrifying that experience would've been. Some estimates put the Roman losses at 6000 to 1, but these figures are highly disputed... though they do give you an idea of how utterly obliterated they were getting.

The 17th, 18th and 19th standards were lost. Rome was shamed. Apparently Augustus never lived it down and was often found mourning the lost up until the day he died.

"QUINTILIUS VARUS! GIVE ME BACK MY LEGIONS!" ... Awesome.

It ended Roman dominion in Germania and pretty much Roman expansion in general.

Germanicus was tasked with retrieving the standards years later and did so, because Germanicus was one of the baddest motherfuckers to ever Don the purple. You don’t get renamed Germanicus Gaius Julius Caesar for eating grapes and having orgies.

Essentially, to answer your question - the Romans didn't have a problem fighting the Germans, they were actually quite successful, having administered victorious campaigns both before and after Varus... But Varus' complete and total defeat at Teutoburg was so haunting, the Romans wanted nothing to do with the place. So much so the 17th, 18th and 19th legions were never risen again, instead those numbers were left to rot in the mud of the Black Forest forever, as a reminder to Rome of its shame.

I also think we see the Germanic tribes as implacable because culturally, the Romans chose to remember them that way - as seen in your Tacitus quote

Edit: I shot a lot of this from the hip from my mobile- can tidy it up tomorrow/others can call me on my bs.

The first thing to be acknowledged (in regards to your quotation) is that Tacitus mention of a “fight for liberty” (duly opposed to the Persian despotism) is hardly a suitable explanation. There are two traditions in Roman historiography regarding the depiction of barbarian people: they can be either (and sometimes, both) (i) animalised or (ii) praised for their unsullied virtue. Tacitus, though he exhibits features of the first, is well-known for having used quite heavily the second process to criticise indirectly the Roman empire (a similar (and more radical) take-on the same theme can be found in an interesting work of the 5th century, De gubernatione Dei, by Salvian of Marseille). The consequence of this is that he tends to mix ethnographical considerations with moral (in its original sense) subtext.

However, his opinion does have some historical ground. The Romans' relations with Germanic people began before the annexation of Gaul, and they were not exactly to the advantage of Rome. Most notably, several legions had been defeated by a massive migration coming from the North, which aggregated several Germanic tribes (the best-known of which are the Cimbri; it also included the Teutones, whose ethnic name has had a surprising fortune). The defeat of Arausio (Orange, in southern France) was particularly humiliating, and resulted in the death of several legions; sources (such as Livy, 67.1) point to a body count higher than in Cannæ. This first encounter arguably gave the Germans a durable aura, even though later engagements were to favour Roman troops (e.g. against Ariovistus: Iulius Caesar, Commentarii de Bello…, 1.50). A recension of victories and defeats is not the good way to think about these events; for the Romans, winning was the normal thing to do. A people that managed to crush Roman troops was a force to be reckoned with.

It is also true that individual Germans were considered to be worthy warriors. It is hard to know what to do of such an assertion; fierceness is a traditional feature of barbarian people in Roman representations, and the more barbarous you are, the best fighter you are (actually, there might be some truth in this idea) [1]. Germanic societies, before they came in contact with the Romans, were fragmented in several small tribes, which must have known a good deal of infighting, therefore guaranteeing a constant military training. It must also be underlined that a good part of the male population of a given tribe must have taken part in these fights; this is the reason why Germanic coalitions succeeded in fielding troops that matched the Roman Empire's, even though their lands were far less populated than, say, Italy or (the province of) Africa.

A final element of importance is that tribal societies were much harder to control and conquer than “states.” The Roman troops had no problem in conquering the Hellenistic west: they only had to topple the local king (sometimes the ruler went as far as bequeathing his kingdom to Rome!) and to replace his bureaucrats by their own (often by recycling members of the former élite). On the other hand, if you vanquish a tribe (or even wipe it out), you still have to deal with numerous neighbours. Indirect control is also harder, because you have to deal with multiple actors. And even if you manage to exercise some kind of overlordship over the area, revolts are probable — much more than in societies where people are used to be submitted to distant powers (a good example for the Germans would be, in Tacitus, the account of the revolt of the Batavi, in 69 AD (Annals, 4.15 and sq. [2]).

Voilà for the Republic and the early empire. The picture is different in later periods: Roman influence (in the guise of subsidies, army recruitment, trade) had contributed to a concentration of power in the Germanic world, and therefore to the apparition of larger confederations. There was still an outstanding military culture amongst the Germans, arguably more threatening than before, but the gradual evolution towards vast entities and the increasing levels of social stratification had changed the situation.

[1] A striking illustration of this view: “the Belgae are the bravest, because they are furthest from the civilization and refinement of [our] Province, and merchants least frequently resort to them, and import those things which tend to effeminate the mind.” (Iulius Caesar, Commentarii de Bello… 1.1) [2] Also famous for being the first account of someone being raised on a shield as a coronation rite.