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.