I have short arms, and I also sew. So I know that my shortness occurs in the upper part of my arm, not the forearm. That said, it depends on the sleeve. If the sleeve is a two piece sleeve, there is a built-in shaping for the elbow usually. If you removed from the top, there is potential to put that shaping in the wrong place - too high up. Also, if you remove from the top of a set-in sleeve, there is usually ease built into the sleeve cap that helps the sleeve fall nicely over the shoulder joint. Shoulders and sleeves - the armscye area - can be tricky to alter, mainly because the body needs to move so many different ways right in that area. So much depends on the sleeve type, how much ease there is in it to begin with, and how well the jacket fits you in the shoulders without the alteration.
