Warhammer 40k lore and storyline
Pathing is bad: units get stuck, bugs include units become unable to move or use skill at all
Targeting is bad: units are stupid, and required manual targeting. Bugs include units doing nothing behind cover
Unresponsive units: click a ranged skill target enemy, nothing happens, they run to the enemy to melee instead - range units tend to kite forward towards enemy
Threw a grenade from bunker targeting enemy, it shows that it explodes in your own bunker, killing your team
Cover system - plenty of space in cover, but only can fit 1 of the 3 units in squad. Runs to the other side of the cover (where the enemy is firing from) to take cover
Gameplay on higher difficulties becomes frustrating
But all these are visibly improved in the retribution expansion - where there is good cover system, responsive units, and good targeting.