In simple terms they arent worth their cost at best (consider same equipment on infantry and SF) SF make up to 75-80% of what a infantry unit can do with same gold spent on them. Those results are actually from the better half of SF the worse have are more like 55-60%
Show me an infantry unit that has 95/95 equipment like my SF do, and I'll concede that point. Until that can be done, I beg to differ.
Smaller units are vastly more efficient than larger units. Thus, a medium size SF force of 40 men can easily rival a 100+ man infantry force. SF also cost less to sustain them over time than an infantry unit. They are very valuable indeed.
Now, if only I had some range 5 SF....that would be sick. (yes I know they exist)