The Roman Empires capabilities are vastly overestimated, in comparison to states like China, which as a civilization and state had a great deal of continuity for almost 2000 years, the Romans lacked a proper civil service and its state institutions were extremely weak. Even by the 1st century AD its quite obvious that the only thing holding the empire together was its military power, which was its downfall. And as soon as foreign tribes began migrating and threatening the already weak provincial institutions in the state declined and then collapsed into warlordism. The Roman Empire as it was lasting any longer than it did in OTL is pretty ASB. Also this idea that Roman legions were somehow these elite super soldiers who could defeat any army on their day is pretty much the result of Roman propaganda. There main strength was the ability to have a tax system which allowed for the existence of a standing army but even then we know that even Ceasar relied heavily on his Gallic and German mercenaries. The empre was able to bully much smaller and weaker oppenents who lacked organized states or military institutions but as soon as the Roman came up against the Parthians and Sassanians we see that the myth of roman invincibility is just that, a myth.