Well, your problem is this: you were told Prometheus was not an Alien film, and not to go into it expecting an Alien film. But you went ahead and went in expecting an Alien film, only to be let down that it wasn't an Alien film.
That is on you.
Prometheus takes place in the same universe, but is not a specifically Alien film.
On its own, it is quite a nice adventure.
(And don't get me started on the fanboys favorite pastime of blaming Lindelof for everything wrong in the known universe. That is really tired, and played).
Judge Covenant when you see it. Not on a trailer.