The necromancers main problem are the undead. and mainly the use of human skeletons.
I think the other 4 heroes (and Sienna herself if im reading the lore correctly) would not approve of using dark magic on humans.
The skeletons also block a lot of the view when summoned. A lot of work has been done to make them less obtrusive but the problem remains.
The solution:
Make Sienna Use only undead ratmen as her minions. Fight fire with fire and so on.
These shorter minions would block way less of the view and help with story progression, separate Sienna from her evil twin.