I'm not going to comment on Sion because I'm really not a Sion main.
But from Sejuani's perspective, I would argue yes and no.
Sejuani has it harder because her core weakness has always been her early game when she doesn't have items makes her a vulnerable target to being caught out in her jungle by aggressive junglers.
It is also hard for Sejuani, if you're building and playing her as a straight up tank. Simply because of the fact that alot of the core armor items have been nerfed/changed.
In my honest opinion, I think Sejuani is fine. But that is because I've changed the entire idea of how I play her which I switched from playing Tank to a Fighter.
Adapting to whats going on in the meta currently and changing up your runes and masteries and build paths is what it is all about and I'm baffled by some that haven't noticed yet.