Is there a disadvantage to the "fan in fin" design seen for the tailrotor on many Eurocopter helicopters? A quick google search will show that a lot of Eurocopter models have their tail rotors contained within their tail booms, while a look at Bell or Sikorsky models (and I'm sure others: I'm using Bell and Sikorsky because they're major non-European manufacturers) shows out-of-boom tail rotors. I can see a couple of advantages to the Eurocopter design:


Less danger: the fan blades can be protected from harm and be further removed from people.
Less noise: I don't have empirical evidence for this, but drawing on knowledge of other ducted systems, this design probably produces less noise, at least in the directions perpendicular to airflow.
Pure symmetry: maybe I just like this from an aesthetic perspective, but unlike the unbalanced external tail rotor design, the in-boom design is perfectly balanced.


Is this design less efficient? Is it the space constraints placed on the rotor by the boom?
The fenestron is a lot heavier than a free tail rotor. This is probably the main disadvantage; helicopters are more sensitive to extra mass than aircraft.

At the time Eurocopter was produced by joining Aerospatiale's and MBB's helicopter divisions in 1992, a follow-on to the successful Bo-105 was under development. That was the Bo-108. Immediately, development was stopped to implement a fenestron into the finished design in order to give all Eurocopter products a common, distinctive feature. This resulted in major structural changes and a heavier helicopter. The result is now known as the EC135.

From this Aerospaceweb article:


  The only significant drawback to ducted fans like the Fenestron is
  that the shroud adds weight that offsets at least some of the
  improvements in performance.


As @Koyovis' answer points out, the low position of the lateral thrust vector is also a clear disadvantage. See here for more.