After asking this question about the Sword of Sharpness, I was presented with this answer which distinguishes between the two main features of the Sword of Sharpness:
Feature 1 (emphasis mine):
When you attack an object with this magic sword and hit, maximize your weapon damage dice against the target.
Thus feature 1 only applies to attacking an object.
Feature 2 (emphasis mine):
When you attack a creature with this weapon and roll a 20 on the attack roll, that target takes an extra 4d6 slashing damage. Then roll another d20. If you roll a 20, you lop off one of the target’s limbs, with the effect of such loss determined by the GM. If the creature has no limb to sever, you lop off a portion of its body instead.
This 2nd feature applies to attacks against creatures and has a 1/400 chance of lopping off the creature's limb.
There is also a third feature to the sword:
In addition, you can speak the sword’s command word to cause the blade to shed bright light in a 10- foot radius and dim light for an additional 10 feet. Speaking the command word again or sheathing the sword puts out the light.
My question: if the first feature doesn't apply to attacks against creatures, then why is the Sword of Sharpness a very rare weapon requiring attunement? Is there something amazing about cutting off a limb that outweighs its low probability of occurring? I am especially curious since there are many benign ways of creating light, meaning that to me the 3rd feature pales in comparison to the first two. But if the first 2 features don't synergize at all, then why is this item so rare (and consequently expensive)?
I ask since compared to other magical items of similar rarity, the sword, should it not synergize, seems a bit underpowered. Consider for example the Flame Tongue:
You can use a bonus action to speak this magic sword’s command word, causing flames to erupt from the blade. These flames shed bright light in a 40-foot radius and dim light for an additional 40 feet. While the sword is ablaze, it deals an extra 2d6 fire damage to any target it hits. The flames last until you use a bonus action to speak the command word again or until you drop or sheathe the sword.
Having a constant 2d6 fire damage seems, from a damage perspective, to be greater than the 1/20 chance of dealing 4d6 slashing damage. This sword also produces more light than the Sword of Sharpness.
A similar concern exists with the Scimitar of Speed:
You gain a +2 bonus to attack and damage rolls made with this magic weapon. In addition, you can make one attack with it as a bonus action on each of your turns.
Having an extra attack to use on my bonus action and consistently having +2 to hit and damage also seems to be a stronger option than the 1/20 chance of dealing 4d6 extra slashing damage or the 1/400 chance of lopping off a limb.
Assuming the above to be true, why then is the Sword of Sharpness a very rare weapon requiring attunement? What am I missing?
If my assumptions or arguments are wrong, please tell me, but to me this weapon seems like it should either have a lower rarity or have the first and second features synergize.
To add a higher level of objectivity, I am comparing both its damage output (no maximum damage against creatures, but 4d6 slashing on a crit and the chance to lop off a limb) as well as frequency of using its ability (1/20 to land a critical, 1/400 to lop off a limb) to those of other magical weapons of a similar rarity.
Best Answer
The Sword of Sharpness has a utility as a magic item that is somewhat variable in that it depends on the player who has it, and on the DM. We'll take the features one at a time.
Light Source
This is fluff. Moving on.
Maximized damage against objects
The utility of this depends on the player and how creative they are in using it. If you never attack an object, this is pretty much useless. However, you can get a LOT of mileage out of attacking objects. Personally speaking, I've busted my way through walls to circumvent an ambush or trap. Cut the hinges off a door so I could remove it without destroying it (the rogue broke the lock while trying to pick it). I've cut ropes to drop chandeliers on my enemies, busted holes in an enemy's boat so we could escape while they sank, and wrecked the DM's death traps by destroying the mechanisms that made it work (he wanted us to go through this complicated disarming process...I worked all the delicate looking moving parts over with a Maul. It worked). Those scenes that turn up where, say...Wolverine carves his way through a wall with his claws? With a Sword of Sharpness, you can do that.
The ability to reliably deal maximum damage to objects, and reliably overcome most objects' damage thresholds if they have one, is really useful in the hands of a creative enough player.
Ask your DM about the environment, look around, think about what you can do with what you can destroy. That door doesn't have to be the only entrance...those light fixtures can be weapons.
Again, this is all stuff you can do with an ordinary weapon...but a Sword of Sharpness is much more likely to finish the job in fewer attempts.
Bonus Damage
Boosted damage on a Nat 20 is nice. Barbarians, for example, are partially built around their boosted crit damaged. And, per the DMG, it's a flat +14 damage on a nat 20, which has more reliability than 4d6, even if it has lesser peak damage output. To address your comparison to the Flametongue...bear in mind that the Flametongue's bonus damage is Fire Damage: one of the most resisted damage types in the game.
Limb Removal
Here is where we hit DM caveat territory. What does it mean to dismember a target? Well, per the DMG, that is going to depend on your DM, and it's going to depend on what you're attacking. If you take a limb off a Zombie or a Troll...no big deal. If you take a limb off something that is living and doesn't regenerate at an insane pace...they are in trouble. Let's walk through what happens here, realistically.
Now, is your DM using it this way? That's DM caveat. But in a game that I am running, limb loss = immediate removal from combat and massive ongoing damage. Unless medical care is administered immediately, the victim of that attack is going to die. Ultimately, in one of my games, the difference between a Sword of Sharpness taking a limb, and a Vorpal Sword taking your head is whether or not you have a chance to save the victim before they bleed out.
Summary
If you don't come up with creative uses for your newfound ability to obliterate inanimate objects and your DM treats limb loss like an inconvenience, then I would agree with you that the Sword of Sharpness probably doesn't deserve its Very Rare rating. But if you get creative, and your DM treats limb loss like the catastrophic injury that it actually is...then it rates quite a lot better. And...+14 damage on a nat 20 is pretty significant (equivalent to average damage of a 13th level Barbarian's Crit damage bonus with a Greataxe), and since it is Magical Slashing damage, very few things are going to resist that.
I would personally rate this on the lower end of the Very Rare magic items...but I still think it deserves the title