Other posts already covered the impact of temperature. I would add that swing covers more water easier. If you know the fish are stacked at certain spot, nymphing would be more efficient.I often read how swinging flies is one of the least effective ways of catching steelhead. It seems most would agree that fishing gear or even beads/eggs under an indicator is much more effective and that guys who swing flies are giving up numbers of fish in order to feel that tight line grab with a fly rod. I am relatively new to steelhead fishing (2 seasons) and my only experience has been swinging flies with a two handed rod. That said, I have watched gear guys and nymph fisherman fish around me and I seem to have as much or more success than most of them. I do focus on fishing rivers that are known to have good swing water and most of my fishing has been for summer steelhead (I'm sure that makes a difference). The "start short and slowly work out line until your at a good distance, then step and repeat through a run" seems to me to be a good way to ensure you are covering all the water. I'm not doubting what I've heard about the effectiveness of other methods over swinging flies (you hear it enough it must be true), but is swinging flies really as ineffective as we're led to believe?