I've only been GMing for a little over a year, and I'm still not fantastic at gauging the difficulty of combat. That said, a couple of the "bigger" NPC fights look... lackluster.
The assassin Risi is the worst offender. Am I missing something or is she a complete dumpster fire? When she goes after the party in Event 5, she is going up against, most likely, a level 4 or 5 party. If they don't have a way to detect her while she's invisible, she gets one sneak attack at 5d6+2 that MIGHT drop someone instantly if she's super lucky and they fail a DC13 Fortitude save. Then she... gives up and goes home? It's that or gently tickle the party with 1d6+2 swings every round.
The encounter with Melisenn at the very end confuses me a bit as well. Is she really relying on a single CR4 monster with less than 50 health to tank a full party of level 6 characters? Her channels do 4d6 which while it'll hurt a little isn't really threatening in and of itself, and she has almost literally no attacking spells. That little byakhee is going to get blown up in one round and she'll just faff about in the sky wriggling her arms for 4d6 until she gets melted too.
My group is definitely a bit more "rollplay" and I'm excited for the Tatterman at the end of book 1 as he looks absolutely deadly, but I'm not seeing the threat in some of these book 2 encounters. What am I missing/not considering?