Useless or redundant units?

[Unwise]

Most of the points I would have made have been made above, but I'll add a few minor ones:

Flying units are not necessarily only used in combat. They patrol much more efficiently than their earth-bound counterparts, they siege more efficiently, and they can be used in raiding parties to strike behind the enemy lines, taking out the comparatively easy provincial defense instead of the main-line armies.

Sacred troops are priced according to their potential effectiveness, not their unblessed stats. As someone above noted, an air blessing on the Guhyaka/Yavana can get them through the rain of arrows to combat where their reasonable defense skill (often much better than protection, especially if you are outnumbered), Awe (!), and high strength can make them effective. There is also the Yavana archer, the only long bow recruitable in the early era as far as I know, with an incredible precision of 12 -- give them a fire-9 bless and you have an astounding artillery capability. Finally, the Guhyaka is recruitable anywhere, not just in the capital, which makes fielding massive, blessed armies possible... and this should also be factored into their cost.