There is no problem with one at a time, except that you do lose time switching from one task to the next for every cartridge. If you are loading for competition regularly, the loading block is going to save you some time, or at least allow you to produce more cartridges in the time you are able to set aside.
The one exception is if you are loading roundballs. With roundballs, you have to kind of roll the ball in glue, then place it in the open cartridge mouth, removing your finger from the ball carefully and moving on to the next.
This can be a challenge with grabbing the next paper, as the fingers tend to get a bit sticky.
The loading block will allow you to make 20 papers, then do the powder, then the balls, with your nasty sticky fingers, before having to wash them.
Excellent charge 20 and seat bullets in block with perfect uniform cartridges
This block is just the right size for my uses. It holds the formed paper cartridges perfectly, and it holds just the right number of them. I don't care to do too many at a time, so, I like having a smaller loading block.