Burning Scythe does the same initial damage that Summer’s Fury does now and did prior to the latest patch in Arena if you disregard the Hero Class magic boost, that isn’t supposed to work in Arena in the first place.
Before the patch Summer’s Fury or Morthani’s Scythe which did the same initial damage (Morthani’s Scythe does even more initial damage now than Burning Sythe and Summer’s Fury) were rarely seen in Arenas in 1.0.8.
I tried them out a couple of times myself but found other weapons ultimately better in the Arena for several reasons.
Now Burning Scythe obviously has some advantages over the other two AoE weapons, one less manacost, dualcolour for easier filling and a guaranteed burn and disease are potentially huge factors, but the initial damage itself is not a problem, once the unintended hero class magic bonus in Arena is fixed.
That being said Burning Scythe is easily the best of the AoE weapons for the reasons i stated and there should be some minor adjustments to either burning scythe or the other AoE weapons, to bring them more in line with each other, be it mana cost or changes to the additional effects of the AoE Weapons.
Anyways i expect balance changes to several weapons once the 1.0.9. dust settles