I don't play Vel'Koz so i have no idea how buggy his ult is.
I do have something to say about Yuumi, i find her Q really unique and fun to use, only problem is that you'll most likely be attached to adc while using it, and if they walk away from enemy champions as they often do as they are ranged champions and they cs from the distance, they basically can cause you to miss and will probably happen often unless they actually walk forward when they see you using it but that can bait them into dying, they need to know when to walk forward so you can land your poke.
What i mean is, her Q is updating based on her position, it follows the mouse in certain range around you, the downside is that if you move after you cast it, your range is only around your champion, it doesn't stay at where you casted it as centar.