I don't see a problem with them, personally. In fact I think I'd quite enjoy playing one. But if I had to guess, people may not quite like the way smite allows a paladin to go nova in ways that other classes cannot achieve. It is a lot of damage, and it is nice that it gets tacked on after the attack is determined to be successful, and it benefits from double damage with critical hits. So in terms of damage potential, paladins have an ability to wreck DM plans and make fights anticlimactic.
If there is any problem with the paladin, that would be my guess. Which again, I think smite is fine. Just like everything in 5e, its a limited resource.