The no-low-cover sentiment is bullshit. Like you pointed out, low vs high cover is exactly 20% so just keep in mind you'll get hit by one more shot out of every five on average. The bigger benefit of being in any cover is removing the exposed crit bonus.

The AWC isn't worth prioritizing either. It costs a ton of supplies for the early game and doesn't contribute much. Each class' standard perk tree is already really good and it's rare that you get an extra ability that your squad can't already do

That said, unmodded XCom 2 is all about not leaving anything to shoot back in the first place. Even if you don't kill everything in one turn there are so many ways to render the remaining enemies harmless:

• Flashbang - cancels psi, disables special abilities, imposes a minor aim penalty. Only for the early game before you get the better options below.

• Frost grenade (Alien Hunters) - if you spot multiple pods in concealment just freeze one, kill the other, then take your time dealing with the still-exposed frozen pod next turn.

• Mimic beacon - build ASAP when you get three faceless corpses. Renders 2-3 enemies harmless and often pulls them out of cover into exposed positions.

• Ranger with implacable and untouchable - if there's exactly one remaining alien you can't deal with, just move a ranger into the open to draw fire (after killing something else to proc the abilities).

• Any number of psi abilities - most notably, stasis freezes alien rulers for a whole turn, letting you set up your squad for next turn and kill any accompanying aliens.