1 - Russia has been using much less reliable (meaning, less-immediately-explodey) cluster bombs when attacking the territory it's now defending, so there's already going to be a huge saturation problem. That die is cast.
2 - If Ukraine perceives that the juice (the temporizing effect using cluster bombs can have) is worth the squeeze (adding more unexploded ordnance to its own territory), seems to me it's their call to make.