Intervention always ends up hurting more than it helps... it fakes demand similar to QE and all other forms of manipulation... it will come back to bite. If it moved up too far or down too far.. tough loss your decision to sign plaza accord creating a deflationary spiral for everyone else other than USD.
I think QE is worse then intervention. QE is determined to be long term manipulation and is very difficult and time consuming to unwind. The effects of QE will also not be immediately known.
Intervention on the other hand takes effect immediately and can easily be reversed by intervening in the other direction (or can simply cease intervening)