That's a good question. I'm going to make some assumptions and assume you mean "Germanic" by Barbarian and Roman as in the Classical period and early Migration period (i.e. before 638 AD), as the Romans considered all non-Romans "Barbarians" and the period after 638 is usually called "Byzantine" and we do have more sourcing from outsiders about that time.
As a whole, we don't know too much, as what "barbarian opinions" about the Romans that come down to us come down to us through Roman writers. Unfortunately, Barbarian authors did not survive, at least not until after the late 5th century. One of the first such authors is Jordanes, descended from the Sadages, who were probably Gothic. His abbreviated Gothic history, however, is a butchering of a more complete Gothic history written by Cassiodorus for Theodoric the Amal, which combines propaganda, myth, and political rhetoric with actual history.
Conor Whately wrote an interesting article on how the "historical account" of the Battle of the Catalaunian Fields is utilized by the Goth Jordanes as a commentary on the political policy of Justinian and Roman strategy in the Gothic Wars. Jordanes argues, indirectly, that Justinian's policies have alienated Rome's neighbors and as such, they had no allies to fight Totila with. Likewise, he argues that Jordanes' commentary shows how he found Roman strategies of maneuver and attrition inferior and unfavorable in comparison to set piece battles, and he saw the Roman Army as weak and effeminate as a result. Jordanes was in Roman employ, so it's also difficult to consider his opinion "barbarian." Barbarian authors like Jordanes and Cassiodorus had limits on what they could say about the Romans, because they couldn't risk alienating relations with the Romans. Especially in cultures that practiced Imitatio Imperii which is the imitation of the Roman Emperor and system in order to maintain legitimacy and power.
We also kind of have a glimpse of how the Huns, or at least a former Roman who had been assimilated into the Huns, viewed the Romans through the author Priscus, a Roman from Panion in Thrace. However, because this survives through Roman writing, it is recorded probably because of Priscus' own political motivations and as a result, is probably biased.
This man, on the contrary, resembled a well-to-do Scythian, being well dressed, and having his hair cut in a circle after Scythian fashion. Having returned his salutation, I asked him who he was and whence he had come into a foreign land and adopted Scythian life. When he asked me why I wanted to know, I told him that his Hellenic speech had prompted my curiosity. Then he smiled and said that he was born a Greek and had gone as a merchant to Viminacium, on the Danube, where he had stayed a long time, and married a very rich wife. But the city fell a prey to the barbarians, and he was stript of his prosperity, and on account of his riches was allotted to Onegesius in the division of the spoil, as it was the custom among the Scythians for the chiefs to reserve for themselves the rich prisoners. Having fought bravely against the Romans and the Acatiri, he had paid the spoils he won to his master, and so obtained freedom. He then married a barbarian wife and had children, and had the privilege of eating at the table of Onegesius.
He considered his new life among the Scythians better than his old life among the Romans, and the reasons he gave were as follows: "After war the Scythians live in inactivity, enjoying what they have got, and not at all, or very little, harassed. The Romans, on the other hand, are in the first place very liable to perish in war, as they have to rest their hopes of safety on others, and are not allowed, on account of their tyrants to use arms. And those who use them are injured by the cowardice of their generals, who cannot support the conduct of war. But the condition of the subjects in time of peace is far more grievous than the evils of war, for the exaction of the taxes is very severe, and unprincipled men inflict injuries on others, because the laws are practically not valid against all classes. A transgressor who belongs to the wealthy classes is not punished for his injustice, while a poor man, who does not understand business, undergoes the legal penalty, that is if he does not depart this life before the trial, so long is the course of lawsuits protracted, and so much money is expended on them. The climax of the misery is to have to pay in order to obtain justice. For no one will give a court to the injured man unless he pay a sum of money to the judge and the judge's clerks."
In reply to this attack on the Empire, I asked him to be good enough to listen with patience to the other side of the question. "The creators of the Roman republic," I said, "who were wise and good men, in order to prevent things from being done at haphazard made one class of men guardians of the laws, and appointed another class to the profession of arms, who were to have no other object than to be always ready for battle, and to go forth to war without dread, as though to their ordinary exercise having by practice exhausted all their fear beforehand. Others again were assigned to attend to the cultivation of the ground, to support both themselves and those who fight in their defence, by contributing the military corn-supply.... To those who protect the interests of the litigants a sum of money is paid by the latter, just as a payment is made by the farmers to the soldiers. Is it not fair to support him who assists and requite him for his kindness? The support of the horse benefits the horseman.... Those who spend money on a suit and lose it in the end cannot fairly put it down to anything but the injustice of their case. And as to the long time spent on lawsuits, that is due to concern for justice, that judges may not fail in passing correct judgments, by having to give sentence offhand; it is better that they should reflect, and conclude the case more tardily, than that by judging in a hurry they should both injure man and transgress against the Deity, the institutor of justice.... The Romans treat their servants better than the king of the Scythians treats his subjects. They deal with them as fathers or teachers, admonishing them to abstain from evil and follow the lines of conduct whey they have esteemed honourable; they reprove them for their errors like their own children. They are not allowed, like the Scythians, to inflict death on them. They have numerous ways of conferring freedom; they can manumit not only during life, but also by their wills, and the testamentary wishes of a Roman in regard to his property are law."
My interlocutor shed tears, and confessed that the laws and constitution of the Romans were fair, but deplored that the governors, not possessing the spirit of former generations, were ruining the State.
Taken from https://people.ucalgary.ca/~vandersp/Courses/texts/prisfr8.html#268 because it would have taken too long to type up Carolla's more modern translation.
The thing with this passage is that Priscus is utilizing it as his own political commentary. So, as a result, it doesn't shed much light on what outsiders really think of the Romans.
I hope this helps answer your question, in part at least. It's unfortunate that we know so little about what the Germanics thought of the Romans, and that Germanic works don't survive. The ones written by Germanic authors that do survive are heavily subject to bias, often pro-Roman, due to both political circumstances and their audience, which would have been largely Roman.
Other potential sources of commentary sometimes come from the Barbarian law codes (Visigothic, Burgundian, Salian, and Merovingian Law) but Late Antique law isn't my specialty. Maybe someone else here can elaborate on the status of former Romans in Barbarian Law.
We also have Persian and Armenian Sources who comment on the Romans, but again, I don't know enough about what they have to say to comment and hope someone else can elaborate on it. To the Romans, the Persians were "effeminate Barbarians." So in a way it is a "Barbarian" opinion.
Sources:
Soby Christiansen, Arne. Cassiodorus, Jordanes, and the History of the Goths: Studies in a Migration Myth. Copenhagen: Musuem Tusculanum Press, 2002.
^ This book is a really good analysis of Jordanes and Cassiodorus, and all the possible socio-political commentaries buried within their "De Origine Actibusque Getarum" and how Jordanes' and Cassiodorus' works correlate to each other. Unfortunately, it is also extremely expensive.