Sidonia no Kishi starts out as a very promising story: you can easily tell how fleshed out the characters and the world around them are going to be, the story is exciting and keeps you on the edge as the characters are facing life and death situations against the mysterious gauna whose motives and origins are unknown.
The main flaw of this manga is probably the pacing. At the beginning, it's just one event after the other, but later on it's the complete opposite: it almost turns into a romcom / slice of life. Is that what I expected when I started reading a plot about fighting unknown invaders? Certainly not.
The second half of the manga is just a pain to read through because of how slow things get. What's more, a lot of plot elements just seem to be completely forgotten and not just minor ones. The fights just lose the edge they have as you simply can't feel the danger any more. For example, weapons that were ineffective against the gauna can now insta-kill them and the death rate is lessened with no given explanation.
By the time you're done with the manga, there's really nothing left of what was there at the beginning. It's like reading an average shonen that tries mixing a plot with many horrible romance tropes and the likes instead. That's a shame because you can tell this could have been much better, if only it had focused on its main plot and tried to enhance it instead of branching out with different themes.
A lot of the panels in outer spaces are also such a mess that it's really hard to understand what's going on and identify gaunas and méchas. Combined with the action, there are a lot of battles where I would have zero understanding of what's going on if not for the dialogue, because you just can't tell what's supposed to be drawn.
All of that makes for a "good" but very disappointing manga that leaves you very frustrated once you're done with it.