Yes there are some disadvantages to IR, but there are some with ultrasonic as well.
Ultrasonic needs to bounce off of a sound reflective surface (like a wall or book, not a pillow).
Ultrasonic needs to be able to hear it's echo. It needs to be fairly direct (not much angle from the surface).
You also need to be careful not to have multiple ultrasonic sensors in the same area, otherwise they will hear each other and get confused.
As far as lighting, the SHARP IR sensors are really good about that (in my experience). It doesn't seem to make a difference if the room is totally dark, or if it's well lit.
Normally you would also need to be concerned about cross talk between IR distance sensors as well, but I've heard that SHARP did an outstanding job of eliminating that (I only have one, so I wouldn't know from personal experience).
IR also seems to be faster than ultrasonic (higher update rate).
I guess there are pros and cons to each method, so you'll need to decide what is best for your situation.
I'm all for gun control... that's why I use both hands when shooting