Its not noted anywhere im just talking based on my observations for my server it may be different for other servers.

You can consider gathering fragments as a threshold most ninjas are obtainable like that. Instead of getting 80 fragments from wheel you reedem them 1by1.

The only ninja not avaliable like that is Hokage Minato AFAIK. Even Hokage Tsunade is avaliable by reedem(too expensive but still its guaranteed).

Edit : even he appeared in hokage optional packs as a guaranteed drop , so basically all ninjas obtainable by RNG can be obtained in more stable way . If you dont want to gamble go for this way of getting them.