Ruin Explorer
Legend
I think we're looking at different playtests. I'm going off the last public playtest from October 14th of last year. Has there been an update released?
Anyway, based on the playtest I'm looking at, Assassinate has two paragraphs. The first gives you advantage against creatures that haven't acted yet. The second says: "If you attack and hit a creature that is surprised, you score a critical hit against that creature. If you use Sneak Attack on such a critical hit, instead of rolling for your Sneak Attack damage, use the maximum result for each Sneak Attack die."
So, yes--rogues who take the Assassination style can reliably use this ability to murder sentries. A 5th-level rogue with Dex 16 and a short sword can assassinate for 1d6+21 damage (short sword crit for 1d6+6, Dex for +3, and maximized sneak attack for +12).
We are - Rogues took a huge nerf going from the September to October playtests I see (I was looking at the former), so actually they're pretty rubbish - the numbers you give are right, and they're far, far lower than classes like Fighters even with 100% SA.
Also, they can no longer "work" for Assassinate in the October one - it's Surprise-only.