The real issue here is that they just implemented Fast Hands as "you get one more bonus action", which is perhaps a little bit more powerful than Larian imagined it would be. In reality, it just adds a few more "Cunning Action" options, and you can only do one cunning action per turn, no matter what it is. That said, if they went with rules as written on this then I'm not sure there's really any reason to take Thief over Arcane Trickster during EA.

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CUNNING ACTION
Starting at 2nd level, your quick thinking and agility allow you to move and act quickly. You can take a bonus action on each of your turns in combat. This action can be used only to take the Dash, Disengage, or Hide action.

FAST HANDS
Starting at 3rd level, you can use the bonus action granted by your Cunning Action to make a Dexterity (Sleight of Hand) check, use your thieves' tools to disarm a trap or open a lock, or take the Use an Object action.