I stopped playing a bit before the cohesion change to lethal, as I had done all 7 levels on lethal and my friend still hadn't brought the game (Shame on him) I returned maybe a week ago and saw a new operation, so was excited. Whilst I am having fun I have noticed a few things that I wanted to point out here. I don't play Chaos missions so don't know anything about bugs etc from that side.

Issues:

1a. Stunlock - Stunlocked repeatedly by Tyrannids attacking just after one another, either leaving me to constantly attempt a parry without being able to counter or to be attacked and stunlocked until I die.
1b. Stunlock - I have been hit out of parries by a warrior attacking slightly after another, abviously on lethal they chunk health causing an inevitable down. At first I thought this was just when Warriors attack and another one uses an unblockable, but this has happened with Blue Parry attacks and the quad whip attack AND normal attacks. 
2. Lethal Levels - Maybe restrict what level a class can start lethal, recently had too many level 9 and lower attempting to be carried through Lethal and it just causes you to either pick a strong class to carry them through or lose. It's not fun as I'd like to play more than BUL/SNI. 
3a. Extremis - Zonethropes (spelling error?) will start up their ranged ball attack during a parry, sometimes without any visual cue (that is noticable) and will hit you out the gunstrike, this also occurs the moment you are not in an animation, so you may leave the Execution to be instantly struck without ability to dodge.
3b. Extremis - Zonethropes Beam attack still hits you despite rolling outside the Beams Visual area. They can also still hit you through walls (Operation 1 reactor walls) despite their other attack being blocked by it.
3c Extremis - This one may be a bug, but I had a lictor animation cancel their Blue jump attack and side step into a quick attack whilst I was locked by the parry animation. If it was intended then very cool, but I have a feeling it shouldn't be able to side step mid air to dodge a shot whilst using that attack.
3d Extremis - Raveners slither Blue attack have weird timing that feels inconsistent, almost like they delay their attack sometimes a little longer. Also, their 3 hit attack should be parried at any one of the hits, to avoid stunlock by other Nids.

Glitch/Exploit:

Operation 1 - On the DEF Reactor section, when you interact with the button a Drop pod appears by the stairs before you are given a reactor number to defend. Standing in the square tile (On the floor pattern) the drop pod spawns (Until the beep sound effect or reactor number is given) will prevent the drop pod spawning completely. Handy for lethal but not really in character for the Nids to avoid crushing you.