It doesn't (really) provide utility

This item is from the section on "Common Magic Items" in Xanathar's Guide to Everything which states this (XGtE page 136, emphasis mine):

The Dungeon Master’s Guide includes many magic items of every rarity. The one exception are common items; that book includes few of them. This section introduces more of them to the game. These items seldom increase a character’s power, but they are likely to amuse players and provide fun roleplaying opportunities.

The common magic items given are not intended to provide utility, but to provide flavor and roleplay opportunities for your game.

The section on ammunition contains this information regarding spent ammunition:

At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.

If I was the DM

As a DM if I was going to add any effect to the Unbreakable Arrow, I would probably just allow the player to round up instead of down when collecting spent ammunition and include that any unbreakable arrows used are collected first.

See illustro's excellent answer for inventive uses out of combat.

It doesn't (really) provide utility

This item is from the section on "Common Magic Items" in Xanathar's Guide to Everything which states this (XGtE page 136, emphasis mine):

The Dungeon Master’s Guide includes many magic items of every rarity. The one exception are common items; that book includes few of them. This section introduces more of them to the game. These items seldom increase a character’s power, but they are likely to amuse players and provide fun roleplaying opportunities.

The common magic items given are not intended to provide utility, but to provide flavor and roleplay opportunities for your game.

The section on ammunition contains this information regarding spent ammunition:

At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.

If I was the DM

As a DM if I was going to add any effect to the Unbreakable Arrow, I would probably just allow the player to round up instead of down when collecting spent ammunition and include that any unbreakable arrows used are collected first.

See illustro's excellent answer for inventive uses out of combat.

It doesn't (really) provide utility

This item is from the section on "Common Magic Items" in Xanathar's Guide to Everything which states this (emphasis mine):

The Dungeon Master’s Guide includes many magic items of every rarity. The one exception are common items; that book includes few of them. This section introduces more of them to the game. These items seldom increase a character’s power, but they are likely to amuse players and provide fun roleplaying opportunities.

- XGtE pg. 136

The common magic items given are not intended to provide utility, but to provide flavor and roleplay opportunities for your game.

The section on "Ammunition" in the player handbook contains this regarding spent ammunition:

At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.

- PHB pg. 146

If I was the DM

As a DM if I was going to add any effect to the Unbreakable Arrow, I would probably just allow the player to round up instead of down when collecting spent ammunition and include that the unbreakable one is always one of the collected arrows.

It doesn't (really) provide utility

This item is from the section on "Common Magic Items" in Xanathar's Guide to Everything which states this (XGtE page 136, emphasis mine):

The Dungeon Master’s Guide includes many magic items of every rarity. The one exception are common items; that book includes few of them. This section introduces more of them to the game. These items seldom increase a character’s power, but they are likely to amuse players and provide fun roleplaying opportunities.

The common magic items given are not intended to provide utility, but to provide flavor and roleplay opportunities for your game.

The section on ammunition contains this information regarding spent ammunition:

At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.

If I was the DM

As a DM if I was going to add any effect to the Unbreakable Arrow, I would probably just allow the player to round up instead of down when collecting spent ammunition and include that any unbreakable arrows used are collected first.

See illustro's excellent answer for inventive uses out of combat.