I would argue that she is all around better than many other common includes that fit into Cube. Feldon being one of them, Magus of the Wheel being another recent one.
Sure, there may be something to say about 3 CMC slots and color, but honestly, I do not think green three drops are good enough to exclude Nissa. Many people run Awakening Zone, and I think she is much better than that card - as there are enough other ways to ramp and optimize curves to include Nissa without losing anything at all.
Feldon and Magus are at least a tier above nissa and maybe more for feldon.
Feldon and Magus are at least a tier above nissa and maybe more for feldon.