My rating:
Medium-lower tier civ
1: Half price encampment is 1/12 of Japanese UA... (1/2 is combat part, 1/2 is the district part, among which 1/2 is the cost reduction, and encampment is 1/3 of that)
also it have +1 housing for encampment, 1/4 of granary.(1/2 housing part, 1/2 bonus)
2: UU: We shall not compare to Pikeman since nobody builds Pikeman. (Or Berserker, etc.)
Impi, 125 Prod, 41 strength, more adj. bonus, extra tech cost, can be upgraded to.
Some comparisons:
Legion, 110 Prod , can fix improvements, with a chance to chop( this values ~40 prod), 40 strength, can be upgraded to
Varu, 120 Prod, 40 strength but actually 45
Crouching Tiger, 160 Prod, 50 strength, ranged.
I don't think Impi is a good match. So this probably do not value high. (Maybe strong at PVP to counter knights, but not useful in PVE games)
Unless the adj bonus from promotion and Impi stack multiplicatively so that it gets +8 per adjancy, if so this unit may be useful, you can use Impi to counter tanks



3: capture city gain corp/army
very, very useful. Like raw +strength for a unit.
(But not as good as Macedon UA)
4: earlier corp/army, cheaper with Ikanda
Does anyone ever made use of the Spanish fleet UA?
5: Garrison provide extra loyalty
This is good. Not sure how good it is.
So its main bonus is "capture city gain corp/army" , "1/12 of Japanese UA", "Garrison provide extra loyalty".