Since you didn’t specify a time period in your question, I’m choosing to interpret it as you asking about the 4th and 5th centuries, though I readily admit that I’m probably wrong. Nevertheless, it does fit the topic of “Germanic” tribes, so I can make do.

So, why were these peoples so difficult for the Romans to defeat? Well, in short, they weren’t, at all. Not even during earlier periods like the reign of Augustus did the Romans really have trouble defeating the various tribes in Germania. Sure Quintus Varus lost three legions at Teutoburger Wald after walking into a trap set by Arminius, but the Romans quickly recovered from this disaster and Tiberius’ adopted son Germanicus won several victories against the Marsi, Cherusci, Bructeres, Chatti, etc. After Roman victories at the Battle of the Weser River (where Germanicus defeated a large Germanic army and Arminius was injured) and the Battle of the Angrivarian Wall, Tiberius chose not to expend any more resources in a conquest of Germania because the Romans viewed it as a largely weathless territory (compared to say Gaul) filled with swamps and forests. So really even as far back as 16 AD its not really a question of why, or how, the people of Germania were too difficult for Rome to defeat, its just that Rome didn’t really see the point in even trying. Now, that’s not to say that it would have been easy for them, certainly the tribes they had already conquered did give them some trouble (of course others proved to be more loyal to Rome than the Cherusci and their allies), and Romans fighting in Germania probably would have been at a disadvantage in the beginning, but nonetheless Rome simply did not see much point in trying. So while the loss at Teutoburger Wald was undeniably a Roman military disaster, its significance is often overplayed in pop culture/history because the Roman’’s recovered almost immediately and spent the next seven years flexing their military might just because they could.

Now, on to what I actually want to discuss, the “Germanic” Tribes of the Migration Age, and their relationship with the Roman Empire. Since you didn’t add any details to your question, that leaves me with a lot of room to work, which might mean this answer ends up being...really long (though I do promise to at least try to exercise self control). In addition, since you only ask the one question, I will be inserting a few of my own questions to answer in an attempt to keep this organized. These will largely be questions I’ve seen on this subreddit before, or that I’ve come across online, but a few might be more rhetorical to serve as a basis for my discussion. Now, with the boring parts out of the way, let's dive in.

So, first why do I keep writing “Germanic”? Well I prefer to use quotation marks because the Romans referred to these people as Germanic because they lived in Germania and had some similarities to one another (largely language wise), but a lot of these people would not have viewed themselves as being part of a larger culture, and if they had obviously would not have used a Latin word to describe themselves. And honestly, we don’t even know where the word Germani came from, we don’t know if it was a Latin word from the get go, or if they got it from the Gauls, or the Illyrians, or the “Germanic” peoples themselves. Herwig Wolfram, a German historian who focuses on late antiquity, believes that the word might be Gaulish in origin, and Wolfgang Pfeifer goes a step further and says that it might be derived from the Old Irish word gair (neighbors), but I’m not a linguists so I’ll just leave it at that. In addition, by late antiquity the only people really referred to as “Germanic” were those along the Rhine, namely the Franks and sometimes the Alemanni.

Other groups were sometimes referred to as Germani by Roman writers but this was more because of how histories were written at that time. It was something of a tradition to refer to groups of people from specific regions by names tied to those regions (I believe this was done to follow in the footsteps of Herodotus but I might be mistaken on that one). For example, the Huns were often referred to as Massagaetae, Scythians, and I believe Cimmerians, because there were no Huns when Herodotus wrote his history but there were Scythians, Massagaetae, and Cimmerians. As an example of this, here’s a brief quote from the 5th century historian Procopius (who accompanied Belisarius during Justinian’s wars to reclaim Italy and North Africa): “the Massagetae outdid all the rest, for they did not even withhold their hand from the sanctuaries, but slew many of those who had taken refuge in them, until Belisarius…put a stop to this.”

​

In this particular example Procopius is singling out the Huns following the siege of Naples because they were already well known in the Roman world for their brutality (or at the very least had a reputation for it). Now, there were no Massagetae in Belisarius’ army (in fact from what I can tell the Massagetae kind of just disappeared from the historical record well before this), but there were Huns, and the Huns just so happened to originate either from the same area the Massagetae had inhabited, or from nearby, and so for someone following this tradition it would be logical to connect them to a Scythian people. So sometimes we have to use context clues to figure out who’s actually being discussed when terms like Scythian or Massagetae are used.

Ok, so this one kind of got away from just a tad, but nevertheless it does still demonstrate the complex reality behind a term like Germanic, and why it isn’t necessarily an accurate descriptor. I prefer to use specific names whenever possible, such as Franks, Goths, Vandals, etc. Though it should be stated that even these names tend to be an amalgamation of related tribal groups. For example Franci isn’t even a tribal name at all, it was just an adjective the Romans’ used to describe the various tribes that made up the Franks, and we aren’t even sure where exactly it came from. The Goths, by the late 4th century anyway, were actually two separate groups, the Greuthungi Goths and the Therving Goths (though the Therving Goths are most likely the older group as they were first mentioned almost 100 years before the Greuthungi). Because of all this complexity, I can understand preferring to use a simple term like Germanic to describe these various groups of people, but history very often isn’t simple and it's important that we understand its complexities. Now on to question number two.