D2 Hackit is safe, all it does is intercept/alter/block/spoof packets (data transfer method, don't ask, most people aren't generally fond of explaining tcp/ip protocols in their spare time, I'm one of them.)
Some modules are however unsafe because they do things which bnet doesn't like (such as tradehack), which results in bnet banning you.
Difficult to explain what "side" d2 hackit is, more clientside I'd say actually, but the results of what it does can effect serverside operations, or sometimes not. Hope that made sense to anyone besides myself.
If it was truly serverside, it would be able to be directly controlling the server, but its not. If it was, we would see all kinds of trainers created (similair to those on open bnet) and any form of a closed bnet economy would be destroyed, almost instantly. And would almost completely destroy the concept of providing entertainment value to anyone besides those who think its fun to not actually play the game. I should stop before I start getting people to read more than necassary of what could be a two sentence reply.