Right now everyone play this GENERAL as an assasin, skipping the summons completely. Except the n00bs that just started and want to try them out, getting owned in the process until they learn the ropes. The ropes = Sednas Yetis Sux.
Why? They simply cost too much mana, and dont give any buffs to sedna in any way. Also, they are melee. Which means they die alot, and are expensive to resummon. Worst of all, they cost skill points, heavy skillpoint investments specifically to even be viable. And even then, they will keep your mana pool drained at all times, as they die faster than your mana regens to resummon.
Lets compare with the other generals summons.
Erebus vampires. These are almost as bad as yetis, being melee and dying alot, except for one MAJOR difference, they are free, they dont cost any mana, and you dont even need to invest a single skillpoint in them. They spawn close to Erebus whenever anything dies, even from level 1 without skillpoints! Also, if you DO invest skillpoints into them, they will eventually become much stronger, with life leach, and you can have 10 of them.
Oak spirits. These guys are similar to Erebus vampires in that they are free(well almost, since totem is really cheap and will summon them all in a few secs if placed correctly). Also, these guys buff oak with more weapon dmg, and oak have abilities that boost THEM as well. Also they fly, making excellent harassers, and since they are so cheap, and dont yield any exp when killed for the opposition, its no worries if they die even to towers, making for excellent chasers/hol interrupts to lock on fleeing demigods.
Queen of thorns. Shamblers are perhaps the best summons, if invested in. They have high hp, are ranged, got good dmg compared to other summons, and they can be eaten for instant healing! Whats not to like? Every QoT get the shamblers.
Summary: Either make sednas yetis cheaper to summon, or give them "something" that no other summon does have, (like for example, a 15% chance to inflict a 1 sec//(0.2 sec dg) stun on attacks) making them at least viable unless you are a complete noob.