I believe part of the problem is is that you're making the wrong comparison.

Don't get hung up on the fact that it makes a skeleton. The verbiage is actually close to Conjure Animals:

The summoned creatures are friendly to you and your companions. Roll initiative for the summoned creatures as a group, which has its own turns. They obey any verbal commands that you issue to them (no action required by you). If you don't issue any commands to them, they defend themselves from hostile creatures, but otherwise take no actions.

In that case, let's look at the stats:

Conjure Animals:

• Can summon eight CR 1/4 animals
• Lasts one hour max
• Takes Concentration
• Uses a spell slot
• Can be used while the caster has 3rd level spell slots and above

Bone of Animation:

• Can summon one CR 1/4 skeleton
• Lasts one hour max
• Does not need Concentration
• Does not need a spell slot (But does require attunement)
• Can be used once per day

So the big differences are quantity, Concentration, and number of uses.

Spell slots are finite, but are not completely limiting. Attunement slots on the other hand are; you get three per character and that's it. Based on level, the character probably already has at least one attunement slot used up if not two. That means the item is fighting for a precious resource.

Concentration is nothing to sneeze at either. That can greatly hamper a full caster (someone most likely to be using the Conjure Animals spell). But on the other hand, you get eight people to act as meat shields vs one calcium shield.

Which means quantity of animals would generally beat out a single skeleton. This puts the magic item on par with a single-use 3rd-level spell. Since the characters are nearing 5th level, that's when your Rangers and Druids would be able to pick up the near equivalent spell. However it becomes available to any class that has a spare attunement slot. But it's not completely unbalanced based on level.

As a DM, I'd hold off until 6th level, just so the Rangers and Druids have their glory.