Watched it 3 times already! No, the Alien is just right (ferocity wise). They are kind of random with regards to how/when they attack depending on circumstances mainly - or in fact storyline

. Sometimes they stalk, almost sniff out and play with a victim, sometimes they rip 6 people to shreds. Often they need to just gather hosts or even feed. Thats kind of consistent in the films, books, comics etc.
I think the best way to look at it, is that this is a Hawley version of Alien, rather than an exact canonical version - which would almost be impossible to produce with the vast and differing, often contradictory lore.
There are lots of issues with this series. However, its a really entertaining show imo. Such a shame it ends next Wednesday, until the new series, which surely there will be going by the huge popularity of it.
I personally would have preferred a more military/xeno centric take, rather than Peter Pan with monsters, but I still love it. 8/10 for me.