Why is NIN considered OP or highly viable?

[Sam26]

Trick attack is essentially an AoE buff like Embolden, Battle Litany, and Brotherhood.

Sry but how is TA in a fight like for example A2s better than the other buffs when they buff all dmg to all adds and not only one?
Buffs always have an advantage compared to a single target debuff when you have multiple piority targets that have to die at the same time. The Value of the Buffs increase in these situations.
The Add phase in A12s is another good example for this aswell as A9s.

yes. Also nin dps tie with mnk.

Nope they don't, if you are reffering to the SSS dummy tests there are some things that aren't accounted.
1. Nin is buffed by TA while the MNK isn't, 20 sec TA uptime is huge in such a short parse with a pot/opener burst.
2. MNK doesn't have his benefit from Brotherhood which increases his dps in a Party (chakra procs).

In a realistic fight MNK would also benefit from TA, his Brotherhood (chakra from partymembers) and his overall better sustain dps.
Immo it's the only job that comes atleast a bit close to SAM while bringing Mantra and Brotherhood for the grp.

So its all based around that one sole debuff?

yes, NIN=TA which is sad in my opinion.