I'd say par-at-best, simply because Xerath and Orianna, as you said, make her feel underwhelming. Part of the problem is that she has a small damage bug; triggering Illumination with her ultimate doesn't include Illumination's AP ratio in the damage.
The other thing is that she has really bad mana issues early-to-mid game because her E costs 130 mana at level 5, which is brutal.
Lastly, and this one is QoL, she cannot "pre-detonate" her E like Gragas can for his Q. As in, she can't activate it again right before it reaches its destination in order to make it detonate on arrival; she has to wait an awkward 0.1-0.2 seconds before she's allowed to detonate her E. FRUSTRATING AS ALL HELL.
She definitely could use a little TLC.