In The Day the Earth stood Still an alien visited Earth with technology so far in advance of ours that we didn't have a clue how it even worked. The same could probably be said for War of the Worlds. In both cases, the arrival of the aliens turned into death and destruction. In comparison, UFO watchers state that UFO spacecraft have been watching us for years. We thnk that the reality is probably somewhere in between.
It is certainly true that if an alien were to come to Earth the level of technology that they would have to possess to do that would be considerably more than what we have. The question would be - why would they visit? There have been many films about aliens coming to Earth to steal water, but water is a very common material in the universe. What is rare on Earth is life and it is that which would draw the aliens attention - either to eat it, colonise it or study it and perhaps even make friends with it.
We don't know for sure, but life is rare and if they go to all the effort of coming here, it must be for an end purpose. Our view is that their arrival would be very obvious and we will quickly know their intentions, one way or another.