Heavy destroyers are an option, but are they the best option? 65 points a model isn’t that bad and they do count as “Necrons” so you are adding to the phase out number. A las cannon + the 12” move gives some awesome range and T5 + WBB is also sweet. Here is the problem with them in my opinion- they only come in groups of three and are a heavy support choice. Three groups limits tomb spyders, or monoliths if you play them, so how many are you going to take? They will get targeted and as soon as you loose one it is leadership test time. Also, is S9 really that good? Not without a melta options.
I’m not saying HD’s aren’t an option, just that they are not the best option for the points and force org loud out. Here is my suggestion and solution with Necrons- once you immobilize a vehicle then forget about it. A land raider is only dangerous for its ability to deliver what is inside- get that unit out by forcing it to walk and it can be killed. Necron players need to shift their thinking from destroying tanks to stopping them and then move on. The only real exception here is with destroyers and S6 shots on rhinos and side armor where volume of shots actually stand a chance of doing something.