It's totally not rewarding. Hypermobile champs have just as much damage, CC, sustain and everything immobile champs have. There is no real penalty for there mobility. A lot of them don't even use mana.
Meanwhile, there is no reward for being immobile. It just feels like a handicap.
Hypermobile champs can engage and escape risk-free even if they made a bad decision. In most cases, even if you CC them mid-dash, they will complete the dash animation allowing them to still engage or escape anyway. This feels like shit as an immobile mage like morg because I'm not being fully rewarded for landed a slow skill shot like her Q.
Overall, there is no real clear reward for being immobile or using mana, its just a handicap.