Is Subaki a bad unit?

He's a decently fast tank. His only issue is the low atk that makes it hard for him to kill outside of special activation. But he is by no means a "bad" unit. People call him bad because their criteria is narrow and is usually not aligned with the unit's specialty.

Subaki likes two skills that were just released but that no one in his right mind would fodder for Subaki, Sturdy Stance 4 and Mystic Boost 3. (For multiple recoveries, you could run Renewal or, if you're not obsessed with having recursive health, a non-recovery B skill: Lancebreaker 3 and the Fierce Stance seal is an option if you think Subaki's speed is defensively high enough and want to exclusively play the triangle game).

If you stack "If foe initiates" buffs on him like Reprisal Lance, Sturdy Stance, and Darting Stance, he becomes an okay enemy phase melee physical wall.

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With all of the above being stated, I think we can say that Subaki is a bad unit. He's not ineffective in his niche and for his availability, but his niche is too narrow.

He's most effective on enemy phase, but not terribly good at player phase; though a good physical tank, he's restricted to countering melees because he likes having a stat-increasing A skill instead of Distant Counter, and of course because of arrows; and (again as a flier) he's limited from running potent B skills that help most fast units pose a threat against at least some units of the color with advantage against them.

He'd have been better as an infantry unit, almost all of his problems stem from being a flier. In fact Nephenee almost is this unit, but as a 5-Star exclusive and with an added PRF that is supercharged compared to older PRFs, it's almost not fair to compare her.

When it comes to flier units, Subaki's main functional competition is L!Robin. Again, it's almost not fair to compare the two, because L!Robin has 2 PRF skills, one of which is likewise "supercharged".

It should be noted that Shigure is also 4-Star available and can be made to be like Subaki with a +Def IV and a +Def refine. With the +HP from a refine and a -Res IV, Shigure has the relevant stats of 45/33/37/27/X. With the +HP of a refine, and a +Atk/-Res IV, Subaki has the stats of 45/29/35/35/X. Shigure can get to 34 Def if he stacks the IV and refine. Then it all comes down to which refine you give Subaki. If he's given +Spd, he'll have 1/1 more Spd/DEF over defense-stacking Shigure, but at the cost of 4 Atk. If he is given +Def, he'll have 5 Def over Shigure but at the cost of 4 Atk and 2 Spd.

Shigure is of course not forced to run a defensive set like this; Subaki is.

Oh no, let me tell you. He is severely underrated. Before I restarted my game, my first ever flier emblem featured him. Nothing too big, just his standard have build with some tweaks here and there. But my god, he can tank. Sure, his attack could be better, but his Ignis bombs hurt like hell. He can tank certain greens and bows if need be he's that tanky. Give him Iotes Shield and bows are literally useless against him. I didn't even switch his Lance at the time, the Sapphire Lance just added more to his tankyness. And with recent lances like the Reprisal Lance and new seasonal lances, I can only imagine the certain builds you could make with him now.
I might just have to start building one soon now that I remember what an underated unit he is.

My Subaki has been a mainstay on my flier emblem since release. While his low atk really hinders him in terms of consistent damage potential, high speed and defense is a cool combination to have. I personally decided to drop his resistance entirely in favor of physical matchups.
I also gave him a Harmonic Lance just for the heck of it. Doesn't have to be optimal as long as its fun.

Fast tank... it's a pity he can't use null follow-up. Bold fighters can give him a bad day. Maybe they give him a prf with NFU on it, then he's amazing.

His biggest issue nowadays is that Catria's weapon is ridiculous and probably overshadows him completely. Her def is almost as good as his when the +3 is active and it's a 16mt (+3) brave weapon for crying out loud.

I think I wait for him to get a prf. But if I were to build him today, I'd look for something like

+atk -HP
Slaying lance+spd
Bonfire
Brazen atk/def
Mystic boost or renewal
Goad fliers (or atk smoke or res tactic)
atk/def2

45/38/37 when brazen isn't active; 52/38/44 when it is. One goad gets him to 56/42, and two would get him to 60/46. If that's not fast enough, change atk/def 2 for spd/def 2 or use +spd -HP