Midlane she seems pretty balanced except her powerful ranged waveclear, heavy cc, heavy damage, and crazy safety.
Toplane, on the other hand...She's a ranged top, with all that brings, but instead of being a normal ad ranged top, she's a mage, so she has aoe wave clear, which makes her even more safe, and means she scales better into teamfights.
Her Q is actually fairly short ranged, unless there's a minion wave present, and that never happens in the laning phase. It also has a high mana cost, so it's considered fair for it to have an incredibly low cooldown for a 0.7 ap ratio ability with a respectable slow to the primary target.
If a melee champion ever actually closes onto her, she presses W and then they don't. Also she gets another free q onto them. Probably another while they back away.
If she's facing Irelia, Renekton, or post six Akali or Kassadin, and pressing W does nothing but 70 / 110 / 150 / 190 / 230 (+ 40% AP) damage + 70 / 100 / 130 / 160 / 190 (+ 70% AP), she presses e, which is not only a ground targeted blink (really really strong for escaping) that can't be blocked or interrupted easily, but also benefits from Leblanc Syndrome thanks to the delay (which admittedly makes engaging harder for her).
Leblanc Syndrome occurs when an ability, such as Leblanc W, or Ryze Ulti, or Lissandra E, has a delayed, targeted blink. The player of the champion insists that the delay makes the ability harder to use, and in some cases, they are right. However, when escaping an enemy without heavy cc or sticking power, the ability says "Pick a location: Either the place I have telegraphed I can blink, or the place I am now. You are only one enemy, you can only chose one location. Ok, picked it? Wrong!". The player of the Leblanc Syndrome champion gets to feel super tricky and clever, often crowing about "THE JUUUUUKES!!!" while only the enemy champion knows that there was no real choice all along and no way for them to win.
Anyway, back to Lissandra. At level 6, she gets to chose between a targeted Sejuani Ultimate that has damage scaling with her primary stat, or a Zhonya's with damage scaling with her primary stat and hp gain.. No, no, not like Kayne or Kha'zix gets to chose, not at all. No, she gets to chose each time the ultimate is up. Whichever one she chooses actually further aids her ability to not have to pay attention to melee champions in the laning phase. In teamfights, she's a huge damage and cc threat with short cooldowns, and powerful utility, which doesn't sound that op until you compare her to the shivering, underfarmed wreck of a melee top that is her counterpart on the enemy team.