Umikkari is an abrasive. Toothpaste also contains abrasive but smaller particle size, along with fillers, fluoride, flavour, colour etc.
Toothpaste is better cos smaller particles do job better ans can deliver fluride.
Umikari the disadvantage is tht its mostly abrasive only. It may give the the illusion of whiter teeth that is only because the particles can be retained in areas between teeth, therefore the contrast. I would say it is also messy. That being said a toothbrush reaches all those places where ur finger cannot.