I don't really see how splitting the slow's duration would really help to distinguish Skarner, and it would only serve to make him weaker.
I actually think there is more to him. The old Skarner was pretty binary where you would insert yourself into a teamfight and just sit there to exert as much CC as possible. Now there's more to be done with positioning and he has the capacity to kite and really pick out targets that he can really kill on his own because he has the damage to be independent.
I see Skarner and Sejuani's situations as fundamentally different. Sejuani is a champion with many inherent weaknesses built into her kit, and her strengths aren't enough to offset the inadequacies. It's difficult to iterate just how bad Sejuani's first clear is. It is really hard to get going early, and her damage has been so neutered from direct and indirect nerfs that clearing camps feels like an almost insurmountable challenge on its own. Skarner is actually probably better at creating picks than Vi (and funnily enough, she is one of his easier matchups to deal with). People will always pick Vi though because she has the capability to react defensively with her wall hop, and there aren't enough measures in place to punish her for making mistakes.
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