How does that even add up? A negative 50% reach?
The TL;DR is you have to reach
past your opponent to actually strike them with a scythe compared to a regular polearm.
As to the full explanation, this will be a bit long, so sorry in advance (and I
deeply apologize if any of it comes off as condescending, that's not my intent). I don't know how much you know about martial arts or historical fencing, but there's a concept called measure (maai in Japanese).
Essentially, it's the distance between two fighters in which one can reach the other with a single step/lunge in order to attack; but there are also different kinds depending on how close you are to the opponent.
They boil down to short, medium, and long ranges; short range is typically grappling range, while long range means you have to take a few steps to engage the opponent. Medium is the measure I described above and is
wildly different depending on weapon type.
For instance, say you have a 6ft tall person wielding a rapier with a 3ft long blade whose arms are 2.5 feet long. Standing in a neutral stance with the sword extended, their reach is 5.5ft. With a single lunge, they would be able to strike someone from roughly
12 feet away, meaning their Medium Measure is about 12 feet.
On the other end of the spectrum, say you have a 5.5ft tall person wielding a katana with a 2.5ft blade whose arms are 2ft long. Because the katana is two-handed, the body is (nearly) squared to the opponent and the weapon held in front, drastically shortening their reach. Their maximum reach is 4.5ft, but in a neutral stance (Chudan no Kamae), their
actual reach is about 3.5ft. With a single lunge and a thrust, they would be able to strike someone about 8 feet away, give or take; with a slash, that range is shortened by about a foot.
See the difference?
Now let's take it a step further and give the second person a battleaxe.
Real battleaxes are roughly 2ft in overall length with a 4 inch cutting surface, not monstrosities like Kratos' Leviathan. Anyway, this would actually increase this person's reach by about 6 inches, as the axe is a one-handed weapon.
However, his actual range would
decrease because many axes aren't designed with thrusting in mind, so you would have to strike with the edge, meaning you'd have to get closer. This would make the now axe wielder's Medium Measure about 7 feet.
With all that explained, let's look at polearms, which can be divided, largely, into two categories: thrusting and slashing.
Thrusting polearms, i.e., spears and lances, are typically held with the dominant hand at the base of the haft and the off hand held about 2 to 3 feet further up; this isn't universal in all situations, but it's most common. Everything of the spear above the off hand is your weapon's reach.
Slashing polearms, i.e., glaives, halberds, and, yes, even scythes, are typically held in thirds; that is, the dominant hand is held about 1/3 of the way down the haft from the blade while the off hand is held about 1/3 of the way up from the base, though the two hands can change places during attacks.
Now, let's say we have a 5.5ft person with 2ft arms wielding a 10ft long polearm; it has a 2ft blade and 8ft haft.
The first polearm is a spear. With the aforementioned hand placement, this makes the spearman's reach
roughly between 7 and 8 feet. Using what we know from earlier, this
should make his Medium Measure 12 to 13 feet, right? Well, yes, but also no at the same time.
See, a spear can use what's called a slip thrust, where the dominant hand slips the haft through the off hand, much like a pool cue. This is quite powerful and doesn't require any movement from the feet, yet affords you an additional 2 to 3 feet of range; if you add that to a lunge, the spearman's Medium Measure can be 14 to 15 feet.
The second polearm is a glaive. With the aforementioned hand placement, this makes the glaive wielder's reach
roughly between 4.5 and 5 feet. Already a significant drop from the spear. With a lunge and a thrust, this would make their Medium Measure about 10 feet. With a slash, it becomes about 9 feet.
Finally, the third polearm is a scythe. With the aforementioned hand placement, and taking into account the blade being perpendicular to the haft, this puts the scythe wielder a reach
roughly between 2.5 and 3 feet. Because the blade is perpendicular, it cannot thrust in line (by which I mean, thrusting forward won't result in piercing an enemy), so you are limited to slashing (or piercing with the point in a slashing motion), which means with a lunge, your Medium Measure would be about 5 to 6 feet.
If the blade were like an axe, with the edge still parallel to the haft. Because the edge is on the inside curve of the blade, you must swing
past the opponent and catch them with it as you slice back towards yourself. This shortens your Medium Measure to only about 4 to 5 feet,
maybe 5.5ft at the outside.
Your only option then would be to choke down on the haft (and I
really hate saying it that way, because of how sexual it sounds; which is why I insist on using "haft" instead of "shaft") like you would a spear. This would significantly extend your reach, by about 4 feet, give or take, but it also limits the mobility of your weapon.
Now, as you can see from all of this theoretical stuff, the design of the scythe makes it a
terrible weapon to fight with compared to any other conventional polearm.
It
does offer some unique techniques in the form of hooking and sweeping or being able to attack from
behind the opponent, but those aren't really enough to outweigh its negative aspects, especially when you have to be within 5 feet of your opponent to do any of those things while a spearman can punch a hole through your eye socket from almost 20 feet away.
Anyway, with flight, you would think the mobility would outstrip those things, allowing it to be more effective and you'd be right, to a degree; the problem then becomes one of aerodynamics and positioning.
Say you want to use a big slash to cut through someone with a scythe. Not only are you making yourself wider, and thus less aerodnymaic, you also have to keep in mind the positioning of your wings and weapon. With a lance like the angels have, though, they can basically just hold it in front of them and more or less become a guided missile.
Plus, you can block a slash from a scythe, arresting the momentum of the wielder, which is exactly why the angels said they didn't use swords; you can't typicaly do the same with a thrust. You
can, with a shield, but then all of that momentum is transfered directly into
you (it's the same reason why bulletproof vests may stop the bullet from penetrating, but you still crack ribs and such).