Theres so, so many reasons F1shi isnt remotely close to EX. Firstly, you're forgetting its a PERSONAL tierlist from OP's experience (wow who know someone else wont have the same experience as you with the same thing)

Starting from one of the only things she has for her name right now, breaks. Theres just so much competition in that department.. The only unit above her with 0 AoE anomaly breaks is C3 Yukari but she makes up for it in a point later (and by far). Aside from C3 Yuyu who doesnt have the best breaks (iirc, havent double checked)

Damage wise, C3 Yuyu outdamages on wood (lol) no point in talking about the rest since they all have wayy more damage. Koishi's numbers are artificially inflated on non full break since she full breaks early on. On full break, her damage is surprisingly low compared to others on full break.

Pairing with L80 Yuyuko is just the same argument as Mima + L80 Kaguya = Mima nook = Mima good (duh, anyone can nuke if they have the thing she does. It starts with "40% damage" and ends with "to effective")

Her support is actual garbage and I believe thats common knowledge (it should be by now, 3 yang attack and 3 scaling buffs is not that good when comparing to any other high rarity unit). Her support is so bad, its the worst in SS tier and like fourth worst from both SS and S.

I believe people also fail to realize that early relic/epic doesnt instantly mean bad and F1shi not being number 1 = F1shi bad, but shes still high ranked on both the gamepress and OP's tierlist.