One main reason that extreme creature control isn't effective against rainbow decks is that rainbows rely significantly on many permanents but can function with just a few creatures, i.e. a graveyard, otyugh and fallen druid. Thats what makes rainbow so scary - you can't play a creature without worrying about it getting killed, but you also can't play a permanent without it getting stolen or exploded. Osiris is just a waiting game until you get a decent shield out and you can play all your permanents fearlessly and just wait for a quint to show up to play an otyugh or something