Two issues I had:
1. If the aliens had discovered earth before we were here and hid their vehicles underground, then come back millions of years later and attacked us, why do that?
If they were here before us, and wanted the planet, why not just stay back then??
What is the reasoning behind finding a planet you want, then thinking, I'll hide my warcraft underground, and if any lifeforms develop over the next million or two years, we'll come back and battle them for it?
2. When the aliens first come down, and wiped out all electrical circuits, clocks stopped, watches stopped, cars stopped, phones wouldnt work, nothign requiring electricity worked, yet people were still able to stand around gob smacked recording it on their video cameras and film cameras when the first big tripod came out of the ground??