The show is definitely a slow burn to build up and like any good show to build character development and suspense, the show does it pretty well. The reviews where they say they didn't see any alien didn't even get far into the series and are probably more into no character development or story and wanting more action. They do show the alien towards the third to last episode and it looks like it was inspired by one of Neil Blomkamp's alien designs in his short films.
It is definitely more about the people and how more of a "realistic" approach of how people would handle situations. Where things slowly unravel as governments figure out what the hell is happening vs a full military response action in some movies.