Who was the one that came up with "stheno tier"?

Other servants can do what she does and better. For example: Elisabeth can boost the party's attack at base level and once she gets her strengthening quest, further increases the damage of female servants. Stheno has the better buff percentage, but you can't deny that there are more female servants than there are divine servants, which makes her better for general use.

Eli can also decrease the defense of an enemy by 20% and that's a SKILL, so that's better consistency. Stheno also has the instant-death chance and charm, but both are targeted at males so again, bad for general use but great if done correctly.

Pair her up with the right team against the right enemies and yes she can be good. But Gamepress ranks servants on how they can perform independently so that's one of the reasons why she's ranked so low.

Waver's better than Stheno is with non-Divine Servants.
Shakespeare is better for Buster-oriented teams.
Elizabeth can contribute to damage while buffing and debuffing.
Jalter works with tons of Buster types while dealing obscene damage herself.

There's a reason Stheno's looked down upon - she's a support in a class that doesn't support. You're absolutely right, that she helps Divines increase their damage (considering her post-Strengthening, which we don't have in NA yet), but her own damage output is bad, her star gen is laughable compared to most other assassins, and the DEF- you refer to is attached to her NP, so it's not as on-demand as something as basic as DEF- can be.

Why bother taking someone who focuses on buffing Divine allies as their only trick, when you can take someone who can buff all types of allies and contribute to actual damage output, themselves? Or buff more than one damage multiplier at a time? Or generate a real amount of crit stars?

She underperforms everywhere as a standalone Servant, and her buffs just don't really compare to other dedicated supports.

By all means, once she hits her Strengthening, use her for Divine teams. But don't try to make her into something she's not.

Its evil to put Stheno on a separated tier, but not Gilles Caster (aka the worst servant of all... by far :v)

I love Stheno and max ascended her (and she is at NP2 for me), but I do have to say that despite being able to Buster-chain with the Servants you mentioned, you do have to remember that the 20% bonus damage from a Buster chain (and the 50% bonus from the Buster first card bonus) doesn't apply to Noble Phantasms, so unfortunately her Buster colored NP won't improve NP chains besides the 20% defense debuff. The bonus damage would only affect normal command cards.

Also, her party ATK buff comes from a Strengthening quest that hasn't arrived yet (should be coming in July)--after it comes, she's got a bit more utility in parties, particularly in buff stacking on members that have the Divine attribute. I'd think it's worth petitioning for her to move up a tier at that point O:

It's totally possible to make Stheno work in your party, but right now she's outperformed by a lot of the cast (even within her own role).

Keep in mind a tier lists point is to compare servants to each other not their viability - Stheno being bottom tier is more about others doing the same things better than it is her being unusable.

People seems to forgot charm lock team exists. She is really good for any male bosses that doesn't have big immunity to debuffs, and those bosses are plenty in the game, one of them even in the upcoming Nerofest. Double Stheno + Euryale or vice versa is an auto win button for those bosses. It's niche, yes, but she is literally unmatched in her niche, that a fact. No one can replace her in charm lock team, ever, not even your so called "male-killer" Orion or Tamamo Lancer.

And for arguments' sake, those who called her def down is inconsistent, Stheno's def down is on her NP, and she has the best raw NP gen on the whole game, so, to some degree she can spam her def down, the same way she can spam her charm in charm lock team.

Honestly, she is good, but people really just loves to follow the way of memes, like how they treat Siegfried.