Manufacturing technology have come so far now so today most manufacturers can produce a great smooth ride. If the ride keep being smooth or not more comes down to the maintenance of the wheel assembly and the wheels themselves.
A few manufacturers excel more than others though and can achieve what we call a glass smooth coaster. Intamin, Mack and B&M are the ones to beat in this area. Sure Gerstlauer, Maurer, S&S etc builds smooth rides but it's rarely on the same level as the above mentioned.
But even with great maintenance a ride will never stay smooth forever, that would require new wheels in a pace that is not economically justifiable. That's why even B&M's gets a tiny bit of rattle after a few years.
When enthusiasts speaks of smooth coasters they usually refers to a ride that only rattles a little bit. A glass smooth ride only exists when they are new.