The assassin just could use any weapon (while the thief was limited to the club, dagger, dart, sling, short sword, broad sword, or long sword), and could use shields, and had a death attack, and could use poison, and had a disguise ability, and kept getting actual hit dice for 15 levels instead of capping out at 10, and still had full backstab, all while still being able to do literally anything else a thief could (if, granted, at roughly -10 percentage points on skill).
Your forgot about learning additional alignment languages. Sigh. Oh, alignment languages. . .