So, I thought in the past Aqua Scepter would have similar damage output to Demon Scythe against WoF, but that was when I thought Aqua Scepter would be able to hit its static iframe cap. But the dust effects aren't hitboxes. Aqua Scepter hits a single target once per 14 ticks, while especially close range, Demon Scythe can reach the static iframe limit of 10 ticks. Shot from below to hit all segments, they do much more similar damage, but Demon Scythe has better reach in this regard so you don't have to squeeze the boss to get optimal DPS this way.
As for stats for Demon Scythe, I'll reference Kepples' DPS tests at
Kepples' Weapon Testing (v0.7.0) . Given the focus on optimal DPS, there is a bias towards melee weapons that benefit from melee speed for 3 damage stats instead of 2, but Demon Scythe consistently outperforms Boomstick and Starfury. Skeletron's a classic, Demon Scythe gets a kill almost twice as fast as true melee Starfury and 50% faster than Boomstick. Against Deerclops, Demon Scythe is more than twice as fast as both. Queen Bee, almost twice as fast as both. Wall of Flesh, 1.5x as fast as Aqua Scepter here and 3rd overall behind Night's Edge and Star Cannon. Boomstick gets Party Bullets in these tests, too, a small edge but not one I ever bother going for over silver bullets. Obviously Demon Scythe crushes the evil bosses, against BoC it's so dominant that it even beats out Sandgun, and that weapon is itself hilariously powerful.
I mostly agree that the underworld doesn't have other relevant stuff for earlygame. I like magma stone, but that doesn't work with demon scythe so that's only really a consideration for multiplayer. Obsidian rose and magma stone -> lava charm are Terraspark ingredients, but much less relevant for experienced players that know how to farm potions. And experienced players are the main people to consider here, as demon scythe is prohibitively annoying to get if you don't either know what you're doing or do the efficient grenade farm I've plugged twice and will plug five more times I love that thing. Even that farm has a decent up-front cost with all the dynamite you need for clearing the area. Also, Demon Scythe would feel a lot less relevant if mage actually had good pre-Skeletron options besides Zapinator and Scythe. And specifically Vilethorn against EoW because that is the third and only other mage weapon that isn't miserable in that fight.
Aqua Scepter's range against WoF is sufficient. In fact, I quickly checked in-game and it's actually higher than I remember, it projectiles reaches the entire screen on my ultrawide monitor.
As for Demon Scythe, there's a lot of asterisks to its very high DPS that haven't been mentioned. First of all:
Ease of use & Mana. Not only is Demon Scythe outright significantly difficult to aim against moving targets, but it drains mana faster than Mana potions restores it, falling in the "Mana Sinkhole" as a result. You either have to outright stop attacking & bleed DPS, build up Mana Sickness & tank your DPS or purposely get hit to trigger Celestial Cuffs' effect. Furthermore, to kill Master mode Skeletron in under a minute with only Demon Scythe, you basically have to facetank phase 1 hands, and at that point it's more of a speedkill than a practical fight you'd do in a normal playthrough. This strategy is also fully outclassed by Volcano + Molten Armor, which will take out the hands as fast if not faster while also taking a lot less damage from the hands. Not to mention that at this stage of the game, the Volcano is drastically faster & non-RNG reliant to obtain compared to Demon Scythe.
I am very familiar with Demon Scythe's matchup against Skeletron because
I used it to kill him 10 seconds faster than Kepples' tests with a slightly less optimized loadout. As for early pre-hardmode, you also have to consider the fact that players will realistically not have most or
all the things that early Demon Scythe is partially dependent on to maximize its usefulness: Mana Flower (-1 accessory slot if you can't mana pot manually), a stash of medium Mana potions, Mystic Robe (Which means giving up on Flinx Coat) and Celestial Cuffs among other minmax-y things. Boomstick and especially Starfury don't scale as hard from optimization in pre-Hardmode, meaning their base performance in reality is comparable to Demon Scythe. Especially when you consider the fact that half of the optimization that Boomstick demands is one of the best accessories in pre-hm anyway (Shark Tooth Necklace).
There's one last asterisk:
Worse compatibility with Minions relative to Melee and Ranged weapons (in pre-hm). I already brought up how Mystic Robe requires you to sacrifice an entire minion slot to give that boost to Demon Scythe, but Melee weapons like Starfury and Volcano can further close the gap with the Sharpening Station glitch, which makes minions most notably deal significantly more damage against high-defense targets like Queen Bee and Skeletron. You can either maximize damage against those targets, or switch out Shark Tooth Necklace for other useful accessories like Brain of Confusion or Bone Helm.
In general, Kepples' tests are offensive/optimization oriented and don't fully represent the actual viability of weapons in the game. Minmaxed Bubble Gun can kill Master mode Empress of Light in under 20 seconds, even without a dedicated hybrid Summoner build, but feats like that alone don't tell the whole story when it comes to general viability. In late pre-hm Demon Scythe is above average in performance, in mid pre-hm it's overall matched by weapons that are far easier to obtain and then in early game it requires so much out-of-the way grinding
AND setting up a "selfish" build around it that it's just not really worth it in the long run.
Now, here are my own Demon Scythe Vs. Aqua Scepter killtime tests against Master mode WoF (No minions used):
The faster Demon Scythe killtime was a result of playing riskily and sticking closer to WoF to maximize Demon Scythe's hitrate. Otherwise, It's the same as Aqua Scepter... as long as you're using Celestial Cuffs, running in a straight line AND getting hit by lasers at least somewhat consistently. If you jump up and down to dodge lasers OR run fast to avoid lasers all the time OR don't use Celestial Cuffs, then Demon Scythe
instantly becomes worse than Aqua Scepter. It's evident that Kepples played hyperaggressively in his tests against WoF, and at that point, close-range Demon Scythe is outshined by Night's Edge + Molten Armor, and perhaps even Obsidian Armor + Minion mixing (Hybrid Summoner most likely outshines that but still).
All in all, Demon Scythe in particular has tons of drawbacks behind its very high power. Availability, mana problems, velocity/aiming and gear dependency all adds up to dragging down Demon Scythe's viability to a high degree, and it's far from being a top 10 weapon. It's still a good A rank weapon regardless. But raw power is not everything, else weapons like Bubble Gun would need to be on this list.