In regards to Sassanid v. Byzantine geopolitics, the issue depends.
Yes, the Sassanid empire has less borders with varying groups than Byzantium. However, this does not mean the Sassanid threats or power is greater long term or short term. To make one statement, Byzantium is simply stronger and more robust than the Sassanid. This is a simple reality and thus all discussions must build from the knowledge that the pain Byzantium can take is possibly too much for the Sassanid and that in protracted wars and conflict, Byzantium is likely to be the winner in such scenarios.
Quite likely, but not really the point.
Byzantium may well win more often than not, but winning will tie up enough of their resources to ensure that any conquests in the west are ephemeral. Justinian was only able to do what he did because the Sassanids were at a low ebb (iirc the White Huns were her big problem) and not in a position to interfere. As soon as Persia recovered a bit, the western gains began to slip away.
Byzantium can certainly survive - esp if it gets a ruling dynasty from somewhere like Syria, who can retain the loyalty of that region - but its eastern border isn't ever likely to be quiescent for more than a generation or two at a time, so campaigns in the west are just throwing away resources.
Much the same would be true for the Ottomans in their turn. In the 16/17Cs, their empire was about the same as Justinian's, but that didn't last. In the 1700s the African lands slipped away, leaving just the core area from Belgrade down to Mecca and Kuwait. But this core was perfectly defensible, and the OE held it w/o major losses until the late 19C.