I read about bike specs online, most make a point to say "sealed bearings". Why is that? I think they are taking advantage of a un-informed buyer who hears "sealed bearing" and thinks this is an upgrade.
The greatest system created for hubs has to be Shimano's loose ball design. They last for 200,000 miles or so with an occasional greasing, are adjustable and virtually indestructible.
Somewhere along the way hub makers thought they could save some money and switched to cartridge bearings. These are fine, I have no problem with them and some of my wheels use them. But they are inferior to loose ball hubs in both performance and longevity.
Referencing a cartridge bearing as a "sealed bearing" is taking advantage of the customer. A "sealed bearing" just means that the cartridge bearing uses rubber seals, as opposed to metal shields. It does not mean the bearing is sealed. Actually sealing a bearing would make it impossible for the bearing to spin.
But by saying a bike has "sealed bearings" it gives the customer a false sense of security, as in "don't worry about the bearings... they are sealed so they'll last forever riding in any condition you can think of".
A cartridge bearing by default has a limited lifespan. Each size is rated for a specific static load. Many hub makers use undersized bearings to save weight. This makes the static load inadequate. This leads to shortened bearing life. Ceramic bearings help you get around this problem but will eventually succumb to it too.
The biggest advantage to "sealed bearings" is that they are easy to replace when the day comes you realize your "sealed bearings" weren't really a selling point after all.