I am more than happy to defend my position but dismissing me as ignorant without specifics is not the way to do that.
The Romans were heavy infantry specialists. That should not be news to you. They excelled at conditioning their troops to be big giant tank men who could butcher people all day long and construct infrastructure on the field - and that's it. And don't get me wrong: boy could they get some mileage out of that! But they never understood cavalry, which meant that cavalry-specialist tribes like the Parthians and the Huns could make their lives miserable. They never understood ambushes, which allowed Hannibal to massacre about a fifth of the entire Roman population in one single battle. And they were TERRIBLE sailors - they didn't understand water, which gave the sea-faring Norse and Vandal tribes pretty much free reign over whatever Roman lands they wanted. The Romans were a one-trick pony that they did very very well, but when faced with an enemy that did not play by heavy-infantry rules, they were absolutely baffled - time and time again, from the days of their humble beginnings to the very last days of Constantinople. They could not adapt to different tactics. Now, where is the lie in what I wrote?
Roman citizenship could not keep up with the needs of the empire. They expanded it when politics forced them to, when they had no other choice - but when the empire stretched from Iberia to Anatolia and tribes who had been living under them for close to a millenium still could not get citizenship, then the loyalty of those tribes to do their part to keep invading raids out of Roman lands just crumbled. A small handful of Roman politicians saw value in turning that around by awarding citizenship to key tribes; famously, Julius Caesar gave the cis-Alpine Gauls their dues for supporting him in the civil war. But the reason they kept on having to hire THEIR OWN PEOPLE as mercenaries was because there was too much reluctance to grant Roman priviliges to people who were serving them but were never fully compensated for their loyalty. Now, where is the lie there?
So let me ask you this: even if you disagree with the conclusions or the perspective of what I'm writing, am I coming across as someone who just doesn't know much about Roman history? Or, wait, you said that I had a TOTAL ignorance of Roman history: am I coming across as someone with a total lack of knowledge on the subject?