Here’s my opinion:
Sandstorm Elementals: Incredibly annoying, essentially forces you to stay in really tight spaces, and generally makes later parts of Zombies more sheltered and less fun in general.
Diabolists: No. Just no. Take them out or take Zombies out.
Necromancers: Honestly, not much of a problem. I’ve never had any difficulties with them at all.
The only map I’ve seen that has any balance even remotely with Diabolists is Cryptic Castle. Necromancers can be a bit troublesome on tighter maps, but they aren’t tough to deal with by any definition. Sandstorm Elementals are nuisances on all maps (with a few exceptions in the case of vertically challenged ones where the sandstorms can’t spawn).