Balance of Harmony is a minor magic item
There aren't many explicit rules in Xanathar's Guide to Everything for categorizing magic items, so I reverse-engineered them in a previous answer.
All non-consumable uncommon items are minor items unless any of the following are true:
- They provide a major combat benefit. For example, they may...
- Grant a bonus to hit, damage, saving throws, or save DCs.
- Summon (or appear to summon) combat-capable creatures.
- Provide in-combat advantage on a common skill, like Stealth or Perception. Removing a disadvantage doesn't count.
- Grant resistance to a type of damage.
- Grant or duplicate combat spells or additional spell slots.
- Grant a flying speed, increase your walking speed, or allow you to walk where you otherwise couldn't.
- Increase one of your ability scores.
- They block high level magic, like scrying.
- They provide a major dungeon crawling benefit, like advantage on thieves' tools checks.
- They provide a major social benefit, like detect thoughts or at-will disguise self.
Balance of Harmony doesn't provide a major combat benefit. Since the Balance only allows casting detect evil and good as a ritual, it cannot be cast in combat. Precasting it is an option, but it takes up your valuable concentration. Knowing the location of a wide range of creatures may help against invisibility, but that typically won't come up more than once or twice in a campaign.
It also doesn't block high level magic and it doesn't provide a major dungeon crawling benefit. It may occasionally be useful to know if fiends are in the next room, but will not help much in gaining surprise. It also may not work at all, if the walls are too thick.
Lastly, it doesn't provide a major social benefit. The social benefits of such an item will vary, depending on the campaign. If disguised fiends are rare (as they are in my experience), the item will provide little to no benefit. In an intrigue-focused campaign, it may break an entire mystery questline. Xanathar's Guide does not provide different lists based on the type of campaign, so I would consider the Balance's overall social benefit to be minor.