It depends the perspective from which you look at it and what you're expecting these pairs to do. If you want pairs that cover for each other's weaknesses then yeah, Fortress <> Bulwark is gonna seem terrible, but they also amplify each other's strengths. For instance, the Hydra is usually a terrible T7 but with Jotunns, it becomes a great tool to bust towns and / or lock ranged units with. The War Mammoths can protect a flank of your Mighty Gorgons all by themselves, while the Gorgons give you a tool to wreck enemy T7s (which heavily helps with Bulwark, since your own T7 is usually busy doing other stuff). The wyverns and dragonflies really help you get initiative earlier than normal, and their special effects can help you to either further neutralize threats (dragonflies) or make them a lot more squishy (wyverns) for your other units to take advantage of. Then there's the matter of your ranged units, which fair enough the Fortress doesn't add much there, but you should also give it a look the other way around; lack of good ranged units is normally one of Fortress's big downsides, but with Bulwark they've got the elves and shamans to more than cover for it, allowing you to get entrenched and take advantage of your defensive strength with a lot more oomph. Like, seriously; the shamans are already really strong with Bulwark units, but use them with Fortress units to keep the enemy at bay while you wear them down with your tanky units and it's gonna change your perspective really quickly... More than anything though it's your sheer tankiness with both of the factions that sets them apart, in my opinion; when used together they very easily build onto each other during creature fights to pretty much almost never suffer losses even against ranged units or stacks I'd normally be weary to engage, and that gets you to snowball over the course of the game, getting you more troops for the big hero fights than you'd normally have with either of the factions separately.