I've had similar problems. I beat Vandals on the bridge over the Danube- using troops from my fort - but the Huns turned up & their first horde went through my spearmen & comitanenses like a knife through butter. Has anybody got a bridge defence lineup that's effective against them?
Morale is how I won all my bridge battles. And fire arrows, which is basically the same thing. I had a very decent general who had some wonderful morale boosts - something like +4, +5 and he wasnt a hopeless drunk or a raving loon. Im aware this mightnt be replicated for everyone, but it does show the benefits of morale.

I lined up a unit of cheap Limitanei across the bridge, with one to either side as the AI tends to push out and around the blocking unit through sheer weight of numbers. You need to keep them pinned in. Had around 2 archer units on each flank. Concentrating all 4 units on one target using fire arrows as they crossed the Huns and Goths were broken often before they even reached my side of the river, soon as they break hit the next with 4 archers, and panic becomes an epidemic. This was on medium battle difficulty. The only real job then was massacaring them all as the Limitanei never broke, and the Huns/Goths broke *through* my lines rather than back the way they came.

Of course, they all "fought to the death" and theres a lot of them so you need infantry reserves behind the Limitanei to plug any gaps that start opening up, and again to keep the limitanei happy that "the flanks are secure".

Also be aware that Hordes have plenty of cavalry that can ford the rivers and will try to kill your archers so keep another spearman unit behind them as reserves when they try to ford.

The only sad thing is my heroic general got a few ideas above his station and ended his career banished to some forgotten part of the empire.

In similar vein everyone seems to use a strategy of only developing advanced military buildings in Constantinople and Antioch. Fine but how do you retrain / reinforce the units on the frontier quickly enough? (Huns arrived one turn after Vandals) Or do you merge weakened units and try to keep a steady flow of new cannon-fodder coming from Constantinople?
Constantinople isnt too far from the frontier with a good road tbh. But whilst you will be getting your elite units from Const/Antoich, cities on the frontier like Sirinum (sp) for example can be used (with low tech barracks) to retrain archers/spears who will be taking the vast majority of your casualties. Your elite high tech stuff shouldnt be taking too many casualties ironically enough because they should be doing the least fighting, thrown in when the battle is about to turn rather than the grindstone on which the enemy exhausts itself. Which is where the easily replacable Limitanei come in.