Ao Kuang God Reveal

I think it's because of how Scylla's weaker than Janus is virtually every area. Being completely outlcassed in:

CC: Scylla has a 30% AoE-tied Slow and a 2.25 sec small AoE cripple/root, both of which are delayed. Janus has a ???-second Banish that isn't a skillshot that hits instantly on top of 40% 3 sec AoE slow + speed buff that also hits instantly. Janus wins.

Damage: Scylla gets 80 bonus magical power and has 260% scalings across her kit. Janus has 210% + 60% from his 3 + 25% from his passive, for a total of 295% - even more of he procs his passive repeatedly, reaching 320% if he procs it only twice, and EVEN MORE if he can get a good snipe. This single skillshot hardly makes him "unreliable," as even without it, he still demolishes people with his 245-270% scalings, and with a good snipe, he brings it up to a ridiculous 470% scalings on his kit. Janus wins.

Basic Kit: Aside from ultimates, Janus has a massive 145% of magical power nuke in his two. Scylla has to pop all of her basic damaging abilities to do 140%, while Janus still has his Portal and Threshold. Janus wins.

Ult: Scylla has more burst damage and potential for a reset. Janus has an unlimited-range teleport plus nuke plus snipe, that increases to 200% scaling with distance. Tie.

Early Game Presence: Janus can rotate to any lane by level five, and can clear the waves easily with his 2. Scylla struggles to clear the wave or have any sort of game presence by level 9. Janus wins.

Mobility: Scylla has a blink. Janus has a longer blink, and a global rotation ult. Janus wins.

Reliability: Janus has unavoidable CC and damage that can't be bodyblocked, but an ultimate that needs CC or skillz to hit. Scylla has a delayed nuke and a skillshot root, and an equally unreliable ultimate. Janus ties with the ult but wins with basic kit.

AoE: Scylla has a massively wide slow/nuke and a fairly wide ultimate and a 3 person root with a tiny radius, but Janus has bigger AoEs on his ult and his 2, and his portal can hit up to unlimited targets with a slightly bigger radius.

Utility: Scylla has MR shred and vision. Janus gets a ******** portal. And longer range. They might tie.

The only things that Scylla has is more magical power, innate MR shred, and a reset ultimate. The MP is worthless since Polynomicon is worthless, making Janus's passive superior. Innate MR shred is also weak since Scylla doesn't worth well with Spear of the Magus, and ends up as a drop in the bucket since every mage gets Obsidian Shard regardless. The ultimate is the only real tactical advantage that Scylla has, but Janus has so much more things over Scylla that he's almost strictly better than Scylla.

She's not low tier, at least in MOBA terms. But the best analogy I have are Pokemon tiers - Scylla would be like Arcanine versus Entei. Arcanine does have a niche, but Entei is still picked 99% of the time over Arcanine due to a better movepool and stats in literally every single stat, making it better in every role except for Atk tank. Arcanine is RU, Entei is UU, making the former low tier, in Pokemon terms.