I don’t know how the damage/armor interaction work, but compared with pretty much any other armor skills, the numbers are something to laugh at.
At level 60, 8* plat, these are all the skills that increase armor in any way (Im not counting elemental armor, like Butter’s plat)
Matador: 2318 armor for 10 seconds (everyone)
Fortress: 1118 armor for 10 seconds (self)
Caine: 185 armor per charge, up to 925 with 5 charges, for the duration of the shield (allies only)
Razorback: 2316 armor for up to 16 seconds (self)
Ritcher: 2553 armor as long as he doesn’t move (self) and 851 armor permanently (self)
Keel: 902 armor to anyone over 75% HP (everyone)
Cast: 288 armor permanently (self)
Vanguard: 9161 armor for 12 seconds (self)
Yanlong: 325 armor per charge, up to 1625 with 5 charges, for the rest of the battle (self)
95 armor under 50% HP (self).
We can see there than Yanlong’s plat has the lower armor increase of all skills, and by a huge difference. Cast plat gives almost triple the armor, without conditions, and also the great amount of 77k HP. Ritcher’s plat give him 9 times more armor, without conditions and also a 5% damage reduction.
I can understand active skills like Matador or Razorback giving more armor than passive, since they need to be casted, can be interrupted or silenced, and only last a certain duration. But the difference between passives is also brutal.
I agree with you, that Yanlong’s plat totally deserves a buff.