I would not suggest Necrons, or orks since they would probably fall into that hard to use catagory with eldar and dark eldar. They require good amy composition,tactics and patients to use correctly. Plus coming off of using SM's you would be tempted to use the other army like SM's and they just don't work like that.
I never knew crons were hard to use. All the cron players I have seen have just blown up tanks with their destroyers and smash up squads in close-combat with a C'tan. Chaos player who's played for more than a decade lost in two turns to a cron force like that, AND the crons managed to destroy my predator with destroyers during those two turns (it was a 12-way free for all). However, I can see how Phase Out would be a problem, especially in 500 pt games.
Back on topic, each army has disadvantages that make me not want to buy them. The disadvantage for IG, Space Wolves, Dark Angels, and Inquisition is the fact that I already have an army that is allied with the Imperium and I'm really bored of having an army that shouts words of blind devotion.
Crons don't have much customization capability.
Orks are big, dumb, and too large in number.
From what i've heard chaos marines are just marines with spikes, extra overpriced troop choices, and different special characters.
And I just DESPISE nids and tau.
So basically I'm trying to find which army ISN'T terrible.
P. S. How is Inquisition not cost-effective? (besides grey knights, of course)