As the game is right now, Chaos feels much more difficult than the Tyranids.  Simple shielded Tzaangors do way too much contesting of the battlefield, Rubric Marines are way too tanky compared to a warrior, and I feel dish out way more damage than they rightfully should.  I also find that both Substantial and Ruthless need to be tweaked a bit more to the power fantasy side of the game.  Enemies all feel too spongey and the risk-reward factor isn't as much there.  Constantly grinding out a mission with the hopes of attaining an artificer-level armoury data point should be more assured.  I am enjoying the game just wish I could play it on more than Average difficulty.

IMO, difficulties are fine. If you want power fantasy, take your relic weapon into easy mode, you will 1 short everything.