The common wisdom seems to be that future pitching performance is harder to predict than future hitting performance. In one sense that's true, but in some ways it's completely wrong.
What is true is that long term improvement is harder to forecast. The vast majority of 20 year old hitters will be substantially better five years in the future than they are now. The same really can't be said of pitchers. Some will suffer major injuries. Others will have more subtle problems with arm strength, mechanics, or other aspects of pitching that reduce their effectiveness. Many will never be better than they are at 20, although some will also reach their full potential later than almost any hitter. So in that sense, the common wisdom is correct. Pitchers' long term development is much harder to predict.
The good news is that in many cases we don't actually NEED to forecast development in pitchers. We can simply recognize changes in true ability faster than others. When a 21 year old hitter with a career average of .250 hits .320 in April, I have no idea if the change is permanent or simply short term variance. However, when a 21 year old pitcher with career ratios of 5.0 for strikeouts and 4.0 for walks completes the first month of the season with a ratio of 35/8 in 31 innings, I can be nearly certain that he's become a much better pitcher than he used to be. If I'm lucky, he'll have suffered enough bad luck to keep his ERA high, obscuring his true talent level, and allowing me to get a bargain.
You can read more of Alex's thoughts on fantasy baseball at waiverwire.blogspot.com.