Can also be really good against Six Samurais.
It stops them from searching with Gateway and making their synchro monsters.
Its 1900 ATK is also good against most of their monsters.
However, Grandmaster easily trumps Raiou with its effect and attack power. If Raiou negates its summoning he can just add Grandmaster back to his hand.
And Magatama can stop its summon negation effect too. Because Magatama is just too strong, it stops Royal Oppression and Solemn Warning too. I think it can stop Solemn Judgment too.