1d10 +1d8 vs 3d6 favors the d6 significantly, with an even greater increase in damage if you are also using a light crossbow. And yes, it is calculating this damage correctly for sneak attack.
They are not rolling 3d6 for sneak attack though. They roll a single d6 and then multiply it, which is a lot worse than rolling 3d6 because you have an equal chance of getting 3 or 18 dmg, when you should’ve had a bell curve.
Rolling more dice is preferable for dependable damage.
