Stationary Mobile:
Because the mobile is stationary, your baby can become bored with it very quickly, my son lost interested in it after only a couple of times. Also the positioning of the mobile can sometimes be a bit awkard for the baby to see, depending on where how he is laying.
Battery Hog:
The attatchment is by far the biggest downside because while it is a great way to get your baby to sleep it ate through batteries like crazy, needing to be replaced nearly every day. So for the amount of batteries you have to use, the environmental benefits do not seem to out way each other. Of course, you can stock up on rechargeable batteries but it ate through these even faster.
To despite the organic materials used to make the bassinet there are probably better choices out there that that would not result in throwing away a ton of batteries and will not make you go insane fighting with the attatchment and having your baby wake up in the middle of the night when it starts making that dead battery whining sound. In fact you may opt for a cradle swing instead that can run off of batteries or electricity to safe yourself a lot of headaches and to save your wallet from buying a lot of batteries.