Assassin subclass thieves are not really much better, just being able to use any weapon, but thieves already have the best in longsword. Assassins are able to use ranged weapons like bows so that is their big advantage with their dex focus making them decent back row archers like fighters can be.
I loved the original 1e Assassin. But here's the thing. A lot of the time, you wanted people to think you weren't an assassin. "Oh no, I'm just a thief." Sometimes, even your own party.
Unfortunately, the whole, "Ima shoot a bow" always gave it away.