The difference is that Abeyance and Orim's Chant are counterable. This is uncounterable. It's fairly ridiculous to cast Yawgmoth's Will, respond with Ancestral and respond with this. There is no way an opponent can come back from this.
If you're casting Will and Ancestral in the same turn, how are you not winning anyway? Once a player has the 7 mana necessary to cast this with those, there's all kinds of ridiculous things they could be doing, like playing Tinker, Will, and Ancestral all in the same turn instead. The fact that this is uncounterable compared to Orim's Chant or Abeyance really only matters if the opponent has 2 counters in hand. If they have 1, they can counter the Chant, but then the spell they're playing goes uncountered. Not to mention that this doesn't have the general utility of Chant (the effect ends more or less right away, it can't Fog) or the benefit of Abeyance (cantrip).
So far, the Split Second effects we've seen have added 1 to the cost of spells. This is true for the Split Second Boomerang and Naturalize. The spells that try to grant uncounterability like Insist or Overmaster cost C, and they cantrip. They can be countered though, so let's say they had Split Second and cost another 1. We're still only at 2C for the combined effect, and it doesn't cantrip. You're spending three mana and giving your opponent a 2-for-1 to give Split Second to whatever you can get on the stack all at once. 99% of the time that seems like it would be pretty horrible.
I'd axe this card for being too narrow or situational way before I was concerned with its power level being too high. It's so incredibly conditional, I seriously doubt you'd find a use for it in most games beyond basically making one of your spells get Split Second.
