You also need to remember, necrons fail at close combat. Even the cc ones. 3rd and 4th ed, this was ok - you had 3 to 4 turns to shoot the crap out of them before you were caught up in cc. Now though, with generally closer deployments, and running, and fleet being more common, pretty much any cc army worth the name will be hitting you on turn 2. They can't consolidate into combat though, so that's some consolation.
Monoliths, admittedly, are hideously op on an individual basis. But people will just ignore them, target warriors, and make you phase out.
Give em a go though, certainly a big change from nids!