I have recently moved from Lord Erebus to QoT, and I like her very much. I do not think that she is underpowered (and she is certainly not overpowered). I simply think that people have to play her properly--she's the only general that can't be really built into a demigod-killing assassin. Like a lot of people have already said above, she takes patience and her best role is support.
She's naturally a spell-caster, so she is very mana dependent --> which means you'll have to build up mana at the expense of building up health, armor, or damage --> which means you have to keep her out of close fights. For me, this means getting the Bramble Shield early and using it often (which means mostly you're in closed mode). For an "aggressive" build, I get the swift anglet favor item (to outrun pursuers and to help out allied demigod faster), one of the mana helms (and a second one as soon as possible), Bramble Shield (upgrade quickly), Ground Spikes (upgrade quickly), Spike Wave (upgrade quickly), Shambler, and Mulch Shamber (for the health boost--it also does AoE damage). But even with this build, she is not going to be killing demigods regularly. Instead, I have her follow one of the heavy-hitting demigods, providing shields for both of us, and I use the occasional ground spikes and spike wave. If the other demigod does his role, he should get plenty of kills, take down some towers, and soak up a lot of damage (thanks to the Bramble Shield). If I do my job, I kill a lot of creeps and minions (thanks mostly to ground spikes and spike wave), debuff demigods, and get a lot of assists.
If you play her as a general (that is, in minion mode), plan to be in closed mode almost the whole time. Plan on controlling one of the lanes, since you are not really going to be supporting other demigods. I do not play her like this often, since I think that she is more valuable as support for other demigods. However, if you invest in Shamblers, Entourage, Bramble Shield, Morale, Idols, and other minion buffs, she can be effective in pushing a lane.
I'm still trying to figure out if Uproot is worthwhile. At least the first level of it does not seem effective, and I never upgrade it from there. Is Uproot's strength mostly dependent upon Compost? Anyhow, I prefer buffing another demigod with a shield and letting him do the tower work. Also, both upgraded Ground Spikes and Spike Wave do decent damage against structures.
The only possible change I would make to QoT is make her shift from Open to Close mode faster. As it is now, she (and Torchbearer) is the most micromanagement-intense demigod. Losing a few seconds between opening and closing can mean the difference between life and death. You almost have to predict what's getting ready to happen, so that you are not caught in Open mode when you should be in Close (or vice versa). So, I would make the change in modes almost instant (or about half of the current time).
Finally, I am reluctant to play QoT unless my allies are good melee fighters (such as UB, Oak, Rook). She can turn Sedna or UB into absolute monsters. However, she does not fit in well with either Regulus or TB as allies. She needs to complement a heavy hitter, since melee is not her role.
