This is a better comparison point, seeing how Smite eats up their bonus action (which, just on the damage front, could just as well have been used to make a regular attack to benefit from Divine Favor's extra damage).
And no, I'm not a big fan of forced Smite conflicting with other stuff they might want to be doing (rather than just giving them an extra spell slot)... but at least it's only one bonus action per day, over Ranger's... however many times you'd need to swap Hunter's Mark around when you have whole subclasses that are built around using bonus actions.