I've been riding in Mechanical hacks for five years, and although I wouldn't let a beginner or anyone who is not adjusted to riding in one use it anywhere near my horse (which is why Indie has a snaffle bridle for friends and the kids, and the one with my hack on it that only I use ;) ), I wouldn't call them cruel. I've seen some horses become quite relaxed in hacks and I have never had a bad experience,injuries, or any sort of "trauma" come from them. It does have long shanks, and generally sits on a sensitive part of the nose so I wouldn't want someone who doesn't know how to ride properly using one.
Any bit can be cruel, some are potentially stronger than others. But I don't see my soft flat nosed, loose rigged hack doing any damage in my quiet hands.
Sides, there is always going to be someone who doesn't like a certain piece of tack you have. I've had people downright say my western saddle was cruel because it wasn't English, and yes I've had people bash my hackamore just because of what it is and not of how it is used or adjusted.