I never did check: Did the later patches also apply to the campaign? I was referring to when I was playing originally, when Ultralisks, mineral cost aside, also took 8 upkeep, i.e. that of 16 Zerglings (who are faster, do like ten times more damage and, combined had more hitpoints as well, so literally their only weakness compared to Ultras was to AoE attacks). I know Ultras were later patched in Multiplayer to a more sane upkeep value of 4 (and I think there was some mineral cost adjustment as well, and minor changes to damage or HP - been too long, though), thus no longer deterring everyone from ever using them. If those patched unit costs also applied in the campaign, that'd make them acceptable tanks.
* Also, while Devourer + Mutalisk is more efficient than just Devourers, Devourers, at least back in 2002 or so, were anything but bad at air defense by themselves - I literally tore down the bases of 3 players in one go with the aforementioned Guardian + Devourer combination, including their scouts and Wraiths and Battlecruisers that they sent to defend.