His ability was shown to work even on things like Endbringers, if I recall - at least, nothing was able to trump it in canon, except for his own greed and stupidity.

While I'm sure a lot of vial results aren't 100% predictable, couldn't he get a good feel as to general results and failure chances?

Considering he's already a part of Cauldron, why haven't they even just called him up, told him to split the timeline, fed someone a vial in the timeline he tells them to, and tells them the results later?

Or use the undiluted vials with high mutation chances to get another Triumvirate-level cape without any of the horrific failures?

EDIT: WoG in the IRC chat. "Simulations break down when it comes to that stuff"

So, the exact means of how it would break down isn't clear, but Coil can not, in fact, predict vial results.