How do you all determine item rarity for home brew items? I know that the DM's Guide gives a basic ideas but for instance when a character is trying to work on a custom item as a quest how would you determine rarity so you can estimate cost and length needed for the item to be created. I know the wording is kind of wonky but I hope I'm making sense. 

Comps. I compare it to other comparable items.

I appreciate the response! Would you say that you'd split the difference if you find different rarities for similar effects? Like if you wanted to make a detection style item similar to weapon of warning (uncommon) or the wand of enemy detection (rare).

I might go with uncommon and add an additional requirement to function, such as an additional Attunement requirement, or specifically keeping the command word requirement. Or I might go rare abut loosen restrictions such as dropping the command word.

That’s where playtesting comes in. Nothing is “ready” until it’s been playtested and it’s ready. When handing out homebrewed items, its always a good idea to attach a “subject to balancing changes” disclaimer to it.

Aside from search filters how important is it really?

I get that an uncommon weapon doing bonus 1d10 damage, granting magic resistance, and flight would sound ridiculous, but what does the designation really do? 

The Rarity determines the “cost” which matters in this instance as the PC is paying to have it made for them.

I suppose...

Since crafting rules are super loose it helps to have an idea on how to balance a quest for time and gold needed to create an item or at the very least provide a mini quests to get the materials for the creation (a la Xanathar's). This particular PC requested to ability to create an item, so it's not just an issue of finding it in loot. 

So this is a rough draft of the weapon that the I'm thinking of giving the Player. It's going to most likely be very rare as they requested a few features. Would love to know if this is balanced.

Blessed Caliburn
Weapon (glaive), Very Rare (Requires attunement)

A bizarre pole-arm that has an extremely long handle that is roughly 3ft in length with a blade of similar dimensions. Created by an Artificer, this weapon has a protective spirit of a totemic creature infused into it. To utilize the full capabilities of the weapon, you must spend one hour forming a bond with the totemic spirit. 

While the weapon is on your person and attuned, it heightens your instincts against danger. You have advantage on initiative rolls and can’t be surprised, except when incapacitated by something other than nonmagical sleep. The weapon magically awakens you if you are sleeping naturally when combat begins. Due to the union between the wielder and the totemic spirit of the weapon, you gain a +1 to Attack and Damage rolls. You also gain the ability to use the weapon as a spell casting focus if the wielder is capable of casting spells.

In addition, you can use an action to cast the Bless spell at first level without expending a spell slot. Once this feature has been used, it cannot be used again until next dawn.

Proficiency with a glaive allows you to add your proficiency bonus to the attack roll for any attack you make with it.