It is actually very unreliable.

Sic Em is a very slow projectile, and similar to Kukulkan ult, you're never gonna hit it on anybody that's either paying attention or is not stuck at a position where they are unable to dodge it. The only times you can hit this reliably is at point blank range and realistically, as Scylla, you don't want anybody at point blank range, lol. This is probably second to Spirit Ball when it comes to hitting it at long range.

Crush can be easy to hit at medium or short range but people can easily walk out of it at long range.

Her ult requires either great aim or set-up to be hit on, again, anybody that is paying attention.

Plus all of her abilities are very easy to Aegis since they are all slow and telegraphed.

With all that said, if Scylla misses one rotation of abilities or is blocked by Aegis, which can happen very easily, she is completely useless in a fight. All she does is damage, damage, damage, and besides Crush, said damage can be very easily missed in which case you will absolutely lose a fight, even if Hi-Rez buffed her ult to do 5000 + 300% of your magical power as damage.

This makes her set-up reliant which is already a huge liability that you don't want on a hyper carry mage. One feature of high tier Gods is they can be placed in just about any comps and Scylla already fails at that. You really can't reliably run her without SOMEONE to set her abilities up.