Well all the visual hacks (esp's, seeing others health, xray, nametags and all that) are safe.
I think kill aura might not be safe because more advanced anti-cheat plugins spawn in fake invisible players, and if you hit the invisible players too much, you get punished.
And all the usual hacks which don't work on servers (like fly, boost, health regen etc) don't use these.
Probably only visual hacks are safe