IMO Multibox is fine, I'd be much more wary of Autohotkey and perhaps Keyclone. Autohotkey is designed to automate tasks, we mostly use it to automate alt+tab, but I have seen WoW scripts that do much, much more.

Keyclone IMO is not automated but I can see where people would/could decide it was particularly the ability to miss out sending some keypresses to some clients, but not others.

While the same functionality is there in Multibox, it is (it appears) to be the case that if you don't send a key it doesn't that key at all, to any client.

Note: I'm not saying keyclone is automation, just that I could see people saying it was. Also note not having bought/used it I don't 100% know the above is true, I'm going by what I have read here.