I was a NICU nurse for many years (before leaving the NICU to do hospital-wide staff development). I never worked any other clinical area. My personal experience working with litterally hundreds of NICU nurses and coaching them on their career development is that it is VERY difficult to switch to another specialty after having been a NICU nurse for more than a couple of years. It is one of the disadvantages of choosing NICU as a specialty as a new grad.
Not only are the patients smaller in the NICU ... but the physiology is different, the equipment is different, the procedures are different, the culture is often different, the mindset is often different, etc. If you leave the NICU after only a year or so, it's not that hard to switch. But the longer nurses work there, the less comfortable they feel going back to care for big people again. The hesitance to switch specialties is most intense in nurses who have little prior experience with big people first. (Note that the previous poster had worked adult ICU prior to working in the NICU.) I've known other NICU nurses who had worked as CNA's as students ... or who had done externships or senior preceptorships in adult units, etc. who were comfortable going back to those units after a stint in the NICU. But note that when they left the NICU, they were "going back" to types of settings they had been comfortable in before.
My student experiences in other areas were limited and I went straight into a NICU as a new grad and stayed there for 16 years (with 2 breaks for grad school). It was the right choice for me and I do not regret it. But for me and many of my colleagues with similar stories, the thought of working in another clinical specialty is frightening because of my lack of knowledge and experience in anything else. While there are advantages in specializing at the beginning of your career, there are also disadvantages -- and that's one of them.