The terminology is confusing, especially when cartridge bearings are sold with or without "seals", but as hubner says, that doesn't make them sealed bearings.
I'm not a complete mechanical klutz, doing all my bike maintenance bar wheel building, but I find cup and cone bearings in hubs and bottom brackets very fiddly to get right. And if you get it wrong and end up with scored cups that can end up expensive and/or difficult to fix.
Tatanab gives around 6,000 miles between maintenance which sounds about right, and that maintenance will be cheap if you do it yourself. By comparison a good cartridge hub bearing can do five or even ten times that before replacement.