I think the 1* reviews are harsh but that's their opinions of course.
Yes there's no special effects, the actress is the main if not only member of the film throughout.
This is very much like Open Water, Buried or phone booth.
If you liked those movies this should be ok.
Sure it's not amazing but it's ok 5.5-6
I really don't know what people expect from films anymore.
Yes there are loads of flaws in the film (being ex forces I worked out a 101 things I would have done differently) but that doesn't mean others would have in the situation.
Give it a try it's not that bad!