I'm going to throw my own two cents here to point some things out.
First of all, in terms of raw damage, Mortal Blow is probably the worst 3p source skill in the game. Even if your character is sneaking, it does less than 300% of your base attack damage. The point of it is to be able to kill anyone under 20% vitality, this lets it do a lot more 'damage' against high health enemies. It is not meant as a starting move.
Secondly, the complete lack of magic armor on the Bone Widow means that it can instantly be cced, and stay that way, possibly by accident. It can never walk through anything electrified, and some cc is much worse than just lose a turn. It is entirely possible to summon it, and have an enemy charm it for two turns.
Thridly, the Artillery Plant does poison damage, and only poison damage. This means that when fighting undead (spoilers, that's often) it is probably worse than useless as it can only heal the enemies.
It is true that they probably should get less of a buff from summoner, but it isn't like they are entirely broken. Although it does seem like the summons shouldn't be able to do as much damage as a character specialized in damage.
