To be quite honest, they are easy to play, difficult to actually win with.
Their "lack of mobility" is not really a weakness, seeing how there are excellent ways around it. It may take some time to learn how to use those properly though. Also, while most units are quite forgiving, there are also a good few weaknesses in the necrons to make up for their forgiving rulebase.
Space marines are a lot more allround, and require a lot less experience/thinking. I mainly notice this in missions however, where that lack of movement really limits me, since pulling units to my lord or monolith doesnt really help much seeing how they only move 6 inch aswell. Especially if the scenario comes down to: race from long edge to long edge of the board, in just 6 turns, hahahahaha, yeah noone seriously expects you to make it with anything but the destroyers, but have fun trying.