So this is the technique Kabuto used in part 1 of Naruto. As you said the user performs a single hand seal and causes the target to fall asleep over a single turn by causing feathers to fall from the sky producing a soothing effect as you said. So the weakness with this technique is clearly the time restraint. It is an incredibly unique genjutsu however as it forces the target to sleep where very few genjutsu can do that. In fact I don't think any custom genjutsu are capable of this since you can't necessarily say how the body will react to feeling soothed when you try to use that.( Nehan Shōja no Jutsu ) - Temple of Nirvana Technique
Rank: A
Type: Supplementary
Range: Short
Chakra Cost: 30
Damage Points: N/A
Description: The user makes one handseal, placing all the targets in the area under an illusion where feathers appear to fall from the sky. This slowly puts the opponents to sleep after one turn.
There is truly not much to say about this illusion. The user performs only one hand seals and causes feathers to call from the sky and the feathers will have a soothing effect on the opponent slowly forcing them to sleep. The thing with this technique is that though it'll eventually force them to sleep, it works very slowly. So you need to wait one turn after it is used before the opponent falls asleep, and the same if used against you. So during that one turn you can easily establish that a genjutsu is being used and break out. Not a very useful technique because of this turn limit.
But nonetheless it is quite unique in that regard. So it is a very obvious genjutsu. There are no jutsu I recall that cause feathers to fall from the sky so seeing it is quite obvious and simply using Kai with the appropriate rank or a full body surge can break you out of it before you fall to sleep. But if they somehow miraculously fail to do that then you've managed to catch them and put them to sleep which means you'll definitely get an easy shot on them.
Its a pretty useless genjutsu but as I said unique nonetheless.