They are completely over-powered with good fighting abilities, hit points, and 9th level spells of any or all types. They can even pick up the highly offensive arcane spells, and out-heal druids.
This reflects a serious lack of understanding of 5e. Bards are good. Not overpowered--just good. They can snag a handful of powerful spells, if they sacrifice their improved melee abilities. They can get improved--not best, just improved--melee abilities, if they sacrifice spell abilities. Bards have long been able to acquire healing, restorative, and buffing spells, so giving them Cure Wounds is mostly a matter of simplifying bookkeeping. (Why have a "Healsong"
and a "Nature's Balm"
and "Cure Wounds" if they all do exactly the same thing?) That the Bard can out-heal the Druid is hardly surprising; Druids have not been healing-focused (capable, sure, but not focused) for several editions now, and I'm not even sure healing was a primary concern of theirs in 2e even.
As for the actual topic of the thread--which isn't complaining that Bards can heal and are thus destroying the "priest" role--I think in general you can't go wrong if you use the following heuristic with some careful thinking.
An item probably does not require attunement if:
1. It's highly situational (used an average of 1 or fewer times in a given session).
2. It provides a "minor" bonus (<5, applicable to a narrow/uncommon range of checks, or has an opportunity cost).
3. It could be justified as excellent but mundane craftsmanship or unusual materials (e.g. +1 weapons/armor).
4. You would be completely comfortable with every party member owning and using it regularly, and these would not be the only ones in the world.
5. The party could find someone, without excessive difficulty, who could make the item for them, for the right price/encouragement.
An item probably requires attunement if:
1. It's a broadly-useful benefit (not in categories 2-4 above, used an average of several times per session).
2. It provides a major bonus (increases/replaces stats, applies to several/common checks, has no cost to use).
3. It enables the user to do something they could not do unless they multiclassed or got a feat.
4. The item is well-known in history or myth, or otherwise has a substantial (and deserved) reputation.
5. You would not expect to see more than one or two, or if the party all had one, they would have all the items of that kind in the world.