Skin/fascia GVHD is extremely limiting and can actually cause patients to become almost completely immobile if it's left untreated. Many patients end up with permanently contracted joints which cause them to be wheelchair bound.
To me, lung GVHD is the scariest of all. Loss of lung function has obviously devastating effects.
Eye GVHD is very limiting, as well - most of us obviously depend on our vision to function in this life.