Hope this isn't already decided...
It's in the Charter.
The item limit balance is the cost. Does your mentioned boon has the same level as the item?
For slots, everyone can carry duplicate utility items or even weapons around. He just cannot use them all at once. Many boons are just utility things that give great flavor but less game benefit (speaking with animals or use a hare as a horse in the feywild book, for example).
Why reduce this?
Another
proposal just passed limiting buying a quiver of duplicate utility items for exactly that reason. And they at least required an action to equip or were effectively impossible to swap(try talking your DM into letting you change your boots or armor in combat - even if he lets you, the action cost would be prohibitive).
I haven't seen the feywild book, but from what I've seen of boons that currently exist, most of them are direct replacements of existing items... except that until this proposal passed they had no slot. They cost the same as the item they were replacing but didn't fill a slot like the item, all the benefit with none of the drawbacks. For example, some of the best Heroic Tier boots in the game (Boots of the Fencing Master - which is mostly a commentary on how few good boots there are) are available as a boon, so you could get them AND another set of boots too.
And what is the problem in this? Wonderous are also slot-less. Do they have to be limited, too?
Was there any problem with alternative rewards that needs to be fixed this far besides of the mentioned item/boon in the first post?
If not, just disallow this one boon instead limiting players to choose between great game effect and great flavor boons.
Wonderous items don't directly replace items that have a slot like several boons do. Wonderous items also rarely give you useful properties AND good encounter powers at the same time. The few Wonderous items that do are the rare exception and are the only ones regularly mentioned in CharOp builds (Enshrouding Candle for example).
I even have a character that has two boons (Dante) - might even be the only one who does - but in looking through the boons, I could see my characters starting to accrue them across paragon, one or two a level, until I had 15-20 of all the most useful ones by epic. 680gp for +1 vs daze/dominate/immobilize/restrain/slowed/stunned and DR5 against OAs? Same for resist 5 psychic and a daily interrupt? Same for a fly speed when you AP and super jumping?
With characters getting into a paragon and/or retiring characters to make new ones, it could quickly become an issue. I'm a CharOp'er and without limits, there's no reason
I wouldn't abuse them without this houserule.