My take on 8th is that they over simplified things. IMO we need a balance between the bloat of 7th and the game my 8 y/o kid can pick up and play right away. In 7th you had characters buffing characters and deathstars ruled the day, with re-rolling re-rolls and stacking bonuses, including movement, to make a blob of unstoppable. 8th has given about every army some sort of first turn wrecking-ball. While I am loving the simplification of their To Hit and To Wound system, the limit of a D6 keeps it in the kids zone for me. They should have at least ramped the dice range to D10, D12, or even D20.
It sucks for us xenos, and it must be difficult for the fluff writers, to make every faction the biggest baddest threat the universe has ever seen. My primary factions in 40k is tyranids, so yeah I'm *&^% pumped for a codex. But our fluff says we may have destroyed a complete other galaxy (or are fleeing something on the scale that did), and that we play attrition games on the scales of planets and systems. Either way that sounds scary on the galactic scale. If we lose a battle, we lost some bio-mass and then fled. But if we win, we lost absolutely zero bio-mass and gain some. We are a faction that can never have a Pyrrhic victory, and that is an incredible feat for any army. But look at us on the table. Since about 3rd edition (when I started playing), the tyranids on the table have not been this at all (except one specific 5th build with tervigon spam). In 8th thus far, we have, just like so many armies, some blender (super melee) unit that just gets sling-shotted into the enemy lines and eats face. My last game was against Tau gun-line and they started as far forward as possible!
So, things change, that's fine and good. Stagnation in a game system kills it. I'm fine with my tyranids changing over time. And the new 8th does seem to put the vast majority of armies at least on a very close playing field to all others. GW will always (always always always) focus on Space Marines and the Cult of the Zombie-Emperor, so they (SM) will always (always always always) have some edge. Depending on where on the spectrum of army/faction you choose, you will just sort of have to accept this. If you are okay with this, and trust in dice being a good leveling of skill vs rules (which is why D6 sucks as it is such a smalllllll amount of variability, especially with those damdable rerolls) or if you are comfortable in your skill as a commander and your strategies, then you can have a wonderful time gaming in the Grimdark future.
If not, we find other, better systems like Kings of War or 9th Age (or go even further with things like Malifaux and Infinity) or go back to things we are comfortable with like 8th (or 7th, which I see signs for posted in some hobby shops!). The toughest part of any new system is waiting for everything (codexes) to be out so we all have updated and complete rules.