Longer is better – it helps stop rattling if the seat post is longer than the shim and banging the seat tube, although if that happens you can always cut down the set post.
As far as safe to use – as long as the shim is at least as long as the minimum seat post insertion required for the frame, in theory you are stressing the frame no more than using the correct diameter seat post. Many frames do require 100mm or more, and many seat post shims are not 100mm.
On carbon I would hesitate – if you spend that much on a frame, you can afford a new seat post, but some posts (e.g. MTB Droppers) come in limited sizes so you don't get much choice: new bike or shim.
Factor in your own weight (I am a lighty) and riding style (I am a wimp on the MTB) and decide how much stress you put on the frame compared to others. Would I use shims? Certainly, on steel and aluminum frames, and probably on carbon if I had no other option. If a 100kg 20yo downhill rider asked if he should shim his off-season Carbon XC race bike to put on a dropper, I think I would suggest not to.
