I'm a big Sejuani enthusiast so I've jumped on PBE as fast as I could to voice my concerns after giving this a few runs. My criticisms will be ranked in order of personal importance, what I'd absolutely like to see changed at the top vs what I can live with at the bottom, and I'll follow up in italics with my suggestions.
Her passive, or rather the freezing mechanic, takes up far **too much** of her power budget, between the extremely high % damage burst tied to it and the mechanic of allowing melee allies to charge it up, in addition to the 2 second stun. Her relatively unique identity among tanks as the most DPS-oriented of the bunch has been exchanged for a lot of up front burst, but despite the insanely high numbers, there's not a lot of enjoyment in it. _I think you should reduce the % damage her passive is capable of and push her damage back into being sustained threat, particularly in her W. Remove the melee allies mechanic; it makes Sejuani's success case far too polarizing on having an ally with attack resets or high attack speed, but offers little counterplay for enemies who will get chain stunned without Sejuani doing anything beyond being nearby. Doesn't feel rewarding on your part either way. _
Her W is the most unwieldy skillshot I think I've had to use. It just feels unnatural and bad; the incredibly low _physical_ damage it does doesn't help. It feels like it's just a tool to build up Frost stacks quicker, instead of being something to use in tandem with it. In addition, the animation feels off to me; it feels too whip-like rather than being a flail. _Give it a little slam on the second swing, purely for satisfaction. Personally, I'd like to see it split up into a two-press ability; W once for the first swing, then you can aim the second W independently, rather than it being all in one go. And make it magic damage again, please. _
Her ult feels very lackluster as an ability, especially when her E is a longer stun and arguably better zone control in the right situations. Not really sure how I would remedy this besides maybe reducing E stun duration and adding it to her ult, but mostly I wanted to highlight this because landing a 5 man ult now is far less impressive and less important than it was before.
I don't care much for the defensive steroid passive being something that recharges out of combat. It's always been something I've found particularly unsatisfactory on other champions like Malphite and Galio; _personally, I think it would be much more satisfying if it was a reward for freezing targets as a quick burst of defense, or a ramping-up steroid that reaches full efficiency after freezing 3 or 4 people or something. _
I'm kind of surprised her Q didn't get changed; being an instantaneous dash, I thought it would have at least become more telegraphed like Galio's.
That's really all the complaints I have right now. Overall I think it boils down to the rework being O-K as far as the kit itself goes, but the power budget is distributed terribly and in a way that feels completely unfitting for Sejuani.