They managed to make Colosssus Slayer passive with parameters, so Sneak Attack should be possible too. Making it a mainhand action only nerfs the Rogue by reducing the effective accuracy of Sneak Attack.
Rogues have gotten the short end of the schtick in BG3 in general. Where is their Expertise feature? It is really important.
Well, now when you mention it ...
Shouldnt that also be toggable? I mean you loose quite huge tactical advantage, if your valuable resource is consumed for first attack that meets the conditions. :-/
Colossus Slayer has never given us the option for not using it. Let's compare Colossus Slayer and Sneak Attack in the PHB:
Colossus Slayer
...When you hit a creature with a weapon attack, the creature takes an extra 1d8 damage if it’s below its hit point maximum. You can deal this extra damage only once per turn.
Sneak Attack
Beginning at 1st level, you know how to strike subtly and exploit a foe’s distraction. Once per turn, you
can deal an extra 1d6 damage to one creature you hit with an attack...
In short, according to the PHB wording, Sneak Attack should be a toggle and Colossus Slayer should be as it is now.