My two pence worth.
If the author has invited feedback, then should be taken at a face value. If s/he doesn't actually want it, they need to remove the comment requesting it. But the key is, feedback should be constructive. If it is constructive, whether positive or negative, then it is a kind gift from a reader who has taken the time to give it. (assuming the author is professional enough to be able to take it)
However,
In the politest possible way, can you offer constructive feedback for them? As indicated by other posters, the novel you describe sounds as if it is rooted firmly in the Sci Fantasy / Soft Space Opera end of the market. This more sounds like you don't like the sub-genre to me.
If the author has purported anywhere on the blurb etc that the book is hard SF, I would suggest limiting yourself to emailing saying you think it may have been mis-listed and misleading. After all, if people are buying it with that in mind and finding it isn't, ultimately it will only result in the book not finding its target audience. That would be valid and constructive feedback. You might also offer that you can provide a critque of the author's science if they so wish.
In addition to some of the excellent selections given above: You may find some of these to be more to your taste:
(I'm assuming you want your SF with a military thrust from the description of the book you're speaking about)
Anything by Peter Cawdron (big big big fan here)
Stephen Moss
Isaac Hooke
(Chrons review for above here:
Quality Self-Published Work Review )
Nick Webb (Chrons review here:
Review: Nick Webb - Constitution )
David Weber (Honor Harrington)
Josh Hayes