An Assault Trooper with Dash, a high speed and 16+ WP can solo most research theft missions in 1 turn due to small map sizes. And you can do this over and over until you have stolen all of that factions tech, while also giving that squad 1500+ experience for doing absolutely nothing besides showing up and walking off with research.
Suggested solution: Some enemies start the game on Overwatch, make the tech theft cost an action (will not fix the problem if the Assault Trooper has 20 WP + Dash OR if you have multiple units with Dash and 12+ Willpower) or make the theft not repeatable at the same Haven until a cooldown has finished.