Sedna
With tweaks from the latest version I think Sedna is powerful, yet balanced. Against a true heavy-hitter like the Rook or the Unclean Beast, she is in trouble, but with her minions she can sustain a constant fight quite well. She is certainly the optimal team supporter. I don`t believe she is unbalanced, I believe she is easy to play. (Because of her reliance on minions and her support auras... very hands-off. It works!)
Queen Of Thorns
Whenever I`ve played her she has seemed quite capable, although like Sedna not quite in a 1-on-1 versus a true heavy-hitter. With the exception of her cast & run structural attack she too seems balanced. She is certainly a Creep-cleaner. Her own special Minions do however - as another player put it - seem to be somewhat weak. When they attack, I might suggest that their underground spikes impart a brief (1 second) reduction to the target`s attack speed and also perhaps movement speed. Perhaps a 1% cumulative penalty, up to a certain finite maximum which could increase as the special Minion ability is improved.
I honestly haven`t tried utilizing her Mulching abilities (to consume her own Minions for Health): I`m not sure what to think of an ability that destroys part of my army for the sake of doing what I could otherwise do with a Health potion. I`m not sure the trade-off is worth the devotion. Maybe Mulch should reward the Mulcher with something much more substantial (Mulch more substantial... hah hah hah... ).
Queen Of Thorns seems parallel to the Torch Bearer or Lord Erebus in that you really have to play her to do well. I still have a great deal of fun with her. Late-game she is not a linebreaker. Structures, yes, Demigods, no.
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Hope such thoughts are of use. Particularly regarding the Queen`s Mulching I`d like to hear other opinions.