Anyone who knows HoTS knows what I'm talking about. Abathur is basically the same concept as Yuumi, attaching to an ally and helping them but the important difference is...
Abathur cannot die. Abathur is always sitting in base. What Yuumi is is basically if Abathur were _guaranteed_ to die when his attached ally died.
This kind of mechanic does work in a MOBA. But not the way they've done it with Yuumi.
Hell, it could even be made to match her lore/teaser even more closely by having the book remain in base while she travels through it to allies. And back when they die.