Although the Nobodies are stronger and smarter than the Heartless, the latter was the only one shown with the capability to destroy the hearts of entire worlds.
If for some reason regular Nobodies wanted to destroy worlds, could they?
Also, were the Nobodies ever as big of a threat as the Heartless?
Although Organization XIII may be more aesthetically pleasing (cooler-looking), the majority of worlds were already destroyed in KHI, whereas not a single world is lost in KHII.
Nobodies crave hearts too, but for different reasons and thus also "treat" the hearts differently than Heartless do.
We do not know if Nobodies can reach the heart of worlds, but if they could they would not try to consume or destroy it like Heartless, they would probably try to somehow capture that heart in order to get "whole".
In the end, for the world in question there might be not much of a difference as the heart is still not were it belongs anymore.
One of the main reasons Yen Sid and co. wanted the Organisation destroyed and the Nobodies defeated was because they were holding millions of hearts captive
and were roaming through worlds trying to create more Heartless and have people succumb to Darkness.
The Nobodies
are as big of a threat as the Heartless, but on a different, less obviously visible layer.
There is of course also to keep in mind that in terms of numbers, the Heartless outclass the Nobodies pretty heavily as only hearts with a strong will actually produce a Nobody. When ten hearts are lost to Darkness and produce a Heartless, probably only four of those produce a Nobody at all.
Human-shaped Nobodies are even more rare, as probably only one of 100 Nobodies is actually made by a heart so strong it produces a human-looking one.
Also would it be in a Nobodies best interests to destroy a world? Heartless just do it because of it instinct, but Nobodies may want to inhabit these worlds and conquer territory.
It wouldn't, as Nobodies want to become whole by gaining hearts, not destroying them.
As they indiscriminately attack the
populaces of worlds in their search though and cause havoc
on the worlds, they can't really be taken much as a "lighter" threat, especially since they are way smarter and stronger than the Heartless.