While Colossus Slayer does not stack whith Hunter's Mark nor is applied to off-hand attacks there is nothing OP about it. Actually I've written simple Monte carlo fight simulator in Python and the most damaging class are now Strength Thief with Handaxes, next is same build but Beastmaster with Giant Spider and finally until he has a spell slots, Wizard with The Sapphire Spark casting Magic Missiles. Hunter are far far away in terms of pure and sustainable damage per round.
That's testing sustainable damage. With a Hunter because you one shot from stealth and range you don't actually get in combat. So sure other classes might be more powerful in combat, but being able to one shot most trash enemies and avoid combat is quite good. One hunter can solo the entire goblin camp from range without taking a single hit and doesn't have to poison them first. It's certainly not bad.
