This issue has been occuring since Lethal difficulty, so even though I am on about the Hardest difficulty, know it's been happening a while so is not "intended for that difficulty setting".

My big issue with the game is that Zonephropes (Spelling?) (Mostly them) and others will now attack during execution and parry animations, essentially creating unblockable attacks that are not dodgeable, like I execute a warrior and the Entire time I am getting blasted by Zonephropes balls and have 2 lictors Jump attacking me throughout the animation. The reason that is an issue is because the AI has been designed to choose who gets to attack, and they shouldn't be able to attack in ways that create unfair and unfun scenarios against the player because they should have what is essentially an attack Token system (I believe, I watched someone who explains AI in games pretty well) to determine who gets attacked. Seems like since 5.0 they have been ignoring it and attacking whenever. mostly caused by shitty Zonephropes

 Also seen a couple of issues of enemies delaying attacks like some sort of DS Boss. Tyrannid snipers not firing when you roll so they can shoot you as soon as the animation ends or Ravagers delaying their blue slither attack so you parry air instead of them.

​You're invulnerable while the execution animation plays out, so it doesn't matter what attacks you. I'm pretty sure you also get some invulnerability frames during parry animations, not sure about that though as I've been playing with block weapons recently. Pretty sure that's been happening since launch though and it's not a new thing. That being said getting attacked while parrying is dangerous but this is an action game, not a turn-based RPG, If enemies took turns like they were waiting in line at the DMV it would be boring and unrealistic. Get better at positioning so you don’t end up as an easy target in melee combat. Sometimes though you just wander into a bad spot, have zero ways out, and kiss the floor. Tough break!

The Tyranid sniper thing, I've noticed it also, but if you see them not firing when you roll, just immediately go into a second roll and they will miss. The Raveners have 2 different animations for their lunge attack, one starts from the same position as their movement animation, this is the fast one, and then a second one where they completely hug the ground and rattle their tail before lunging, this is a bit delayed so you want to wait about a second before parrying. It's just something that you'll learn to spot the more you play, so just stick with it!

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Invulnerable during the animation yes, but they are attacking constantly so you have no time to react once the execution ends.
Enemies within the coding wait for their turn, its why you don't get Jumped by every single minoris at the same time. Whilst I am not asking for Turn Based I am asking they respect the games coding and logic. 
Go immediately into a second roll they may not fire, choosing to only fire after the 3rd or 4th Roll, instantly as soon as you get out of the roll animation. This is uncounterable as they hold until the frame the animation ends. 
Raveners is a good shout, I'd say they probably kill me the most from my own skill issue. although I have beat all ops on Lethal (Before the Difficulty decrease) and most on the new one, so I have a good grasp on the game, these are just issues that I have noticed that are actual issues. Most came when multiple Extremis appeared at once, but some are related to Warriors too.