I'm starting to remember why I never finished playing the Necrons on Soulstorm. They're so darned inexorable I never really get the feeling of being challenged. While the Orkz are really strong, they do take a bit of time to build to the point where the best units are out, but the Necrons are deadly from the start.
I'd like to finish playing through with them this time out, but I'm not sure I will. I keep wanting to play the Guard again...
The Guard are a lot more vulnerable, which is what I like about them. Their whole strategy really is to desperately entrench until the tanks turn up to win the fight. The Sisters of Battle are tough, but they go down to enough heavy weapons emplacements just like everyone else. The Guard can mass crazy amounts of firepower if you set them up right. The hardest part of playing them, I think, is that they are the only faction that are not 'infantry-centric' - and more, they are actively 'vehicle-centric'. If you try to use the guard the same way you would use the Orkz or the Necrons, you'll get wiped out every time.
The Sisters are a superb anti-infantry force with all their flame throwers, but the Guard vehicles are vastly superior. Use a Vindicare Assassin as spotter for your Basilisks and destruction reigns over all. Those things have a range that's almost beyond belief.
The Guard are a 'slow creep' force rather than a 'storm force'. Move in and fortify everything and you won't be beaten easily.