It says challenging, and it delivers. If your group is a particularly casual, friendly sort, this may not be your mission. Failure is likely to result in a player (or three) getting killed, but there are plenty of ways to recover before that happens.
Still, this is for late in a campaign. No pre-built characters helps further that. This mission focuses around an especially risky information-gathering sequence, and will almost certainly be followed with an intense combat encounter with Guild forces. Not for the faint of heart, in either the GM or the Players. I've never been one to think that the GM should be fighting the players, but if you want some competition, this adventure will do the job. The TN numbers are high; double-check your players' stats before feeding them to the witch hunters.
Seriously, I hope your players aren't too attached to their characters. If the deck's not on their side, they may be overwhelmed by enemies. It's not a combat-only encounter, but there will be gunshots.