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Yes bone dance does leave the target vulnerable and able to be wailed upon for the duration, as long as the Nethermancer keeps making the effect rolls.
Notably in 3rd ed, making this work, most of the time, requires use of willforce every round.
Other than that, the main repercussion to the Nethermancer is severe risk of Alcohol poisoning.. from all the drinks his buddies buy him for keepinmg the horror helpless.

Yeh, your free kick them while the nethermancer holds them down.

Not your imagination.
Paying once is the correct way to do it.

(Normally, there's a rule of thumb that we put into widespread use in ED3 where if you roll something, you pay its cost. It was a concept we wanted to communicate to the players. So that was what I had in the back of my head when I said it's Strain every time. But you don't actually roll Willforce when you make an Effect Test. When you cast a spell, you determine a new Step based on Willforce, and then that becomes "locked" into the spell formula, and the spell becomes independent from the caster. It's this "locking into" that is the use of Willforce, not the subsequent Effect Tests. Just the same, when you have a spell with a Duration based on Rank, and you later get a bonus to the Rank, the Duration stays the same.)

I think it prevents them taking actions but I am a 5 hour drive from my books and I could be thinking 2nd edition

I'm pretty sure the last sentence in the spell description (If failed, the target regains control that round—he may move and act of his own accord.) means that as long as the Effect Test is successful, that the target can make no actions.

OK. Could we please get some (semi)official ruling on this? Or is it better to start a new "Bone Dance" thread?
I think it would be too powerful for a 1st Circle spell to effectively mimic the effect of 3rd circle "Pain" (no action possible) so easily. I know that casting Bone Dance takes time, but still...

S.

Would be nice, in our campaign bone dance isn't very usefull even if successful (IMHO) while no action possible would make it very much so..

Its the "he may move AND ACT of his own accord" that suggest a successful test means he cannot control his actions..but we don't play it that way in our campaign..

Bone dance is just about the movement.
(Otherwise, there wouldn't be a need for the Harried modifier -- if you can't really act on your own accord, the -2 to your actions isn't necessary.)
With the spell, the caster's main "cost" is giving up his Standard Action for concentration, making the target effectively loose his MR, plus making him Harried.
You still can make the target pretty helpless with this if you just stay at range and shoot, and the target can't close in...