If T'au are that purely good, then it help justify IoM's cruelty. T'au empire is so tiny that it can be squashed by any faction. That's why you need to be cruel to survive.
The empire also proof that IoM is needlessly cruel, which make it grimdark as well. You need a light to know how dark it's.

It's the best in the trilogy. Only thing I dislike about it is that puzzles are so easy and short. Sequels improved on this aspect but too gun oriented.
Transversal with ropes in this one I way more fun than the 3rd one as well.