Actually, WisTrit is right; if the Stalker's ability is an overpowered perk, then the invisibility of the Witch is just absurd. I tried playing in different ways, and here and there. By the time the upgraded Witches appear, invisibility is almost useless. In the endgame, it's just a nice feature, good for a single retaliatory strike, but no more. And about the piercing attack... what prevents you from still deploying only Stalkers? Or, in general, stop scrimping and fighting with a normal, large army?
Oh yes, the Witches only shoot at half the battlefield, which means they need two shots for full damage. Well, for full damage in one hit, our Mistresses also need at least two turns, the only difference being that anything can happen to them along the way, up to an encounter with the Devil, while the Witch will deal damage already in the first turn.
But all these arguments don't have a solid basis, since Witches are usually used to cast (de)buffs. You can say something now about splitting the Witches into single units (as I do), but if we're talking about the endgame, there are usually no more than two free slots, although sometimes that's enough.
And now imagine - they go invisible, and the attacks start targeting the Riders. Unpleasant, isn't it? Of course, the Witches are "more expensive and stronger," but the Riders are still tougher and can withstand even the strongest beatings, and you, of all people, Johnny, know that Riders are an indispensable unit in the final battle, very useful and strong.
The Mistresses, indeed, move very, very slowly, even the alternative Gargoyles don't move that slowly.
So, invisibility is great, but not so great that you should fanatically use it everywhere.
If I have a choice - to lose a couple of Hunters during a piercing attack, or to lose an extra five minutes painstakingly arranging, moving, and calculating all sorts of possible paths for the Witches, I'll choose the first. I love having fun, and that's what it's all about.