Odin, you didn't take into account the fact, that Creepy Fog expires at the end of the turn. So if you pack your high class minions and/or master behind it and lose the next initiative then... you are in trouble. Especially against crews with multiple Companion options. This is a huge gamble which is very fluffy with a Stitched but it's hardly a game breaker tactic.
Also there are many possibilities to deal with this. Hans is just one. Young and Mature Nephilims don't need LoS to make a Diving Attack. (You can even save your Mature after killing the Stitched with Lilith's Transposition or Lilithu's Lure or Zoraida's Obey.) All horsemen can make ride-by attacks against it. The same is true for the Coryphees or the Ronin. These are examples just by hart. I'm sure there are many more options.
Also, if you have a terrain piece on the board with Ht4, if you climb/fly/teleport on the roof with a Ht2 model, you do have LoS on anyone behind the fog.
I don't want to say the Stitched is a bad model, not at all. What I want to say that they are not unbeatable and they are not more underpriced than a few other models in the game. (Rotten Belle for 4 SS? Yes, please! Grave Spirit for 1 who can cast Molly's instant killing spell with SS use? Hmm...)