Managed to run a few test encounters with my table group. Arena style, 50ft standoff.
Cleric 1 / Stormlord 5
Warblade 1 / Bloodstorm Blade 5
Knight of the Weave 6
Goliath Fighter 2 (dungeoncrasher) / Barbarian / Something / Broken un-erratta Dragonspawn (flying charge bullrush ubercharger, ultimately a somewhat invalid build)
Threw them at some CR 8 - 10 encounters.
Dybbuk - PCs got destroyed, could not easily overcome DR 10/good, some trouble with SR 21 (magic missile missing 1/2 the time), trouble with several energy immunities (energy weapon crystals did nothing), trouble with incorporeal miss chance. At best they had it at 50% of its hp. ~7 rounds in at the surrender point it was 85% healthy with the KotW dead with its flesh being worn by the Dybbuk.
Fossil Megaraptor - took them a while to kill it due to DR10/adamant, but its attack bonus of +7 was non threatening to their 22-30 AC ranges, despite the damage potential if it hit.
Rage Drake - Pounced the KotW, severely damaging him. After recovering, KotW made the mistake of getting back into melee where he got grappled and killed.
Quetzaloatlus - killed monster in 2 player actions, since ubercharger went first. Would be more threatening starting from its natural flying advantage.
Yuan-ti Assassin - Detected by the KotW via constantly buffed see invisibility, then purged by the cleric. Defeated quickly after being exposed. Pretty weak in a straight up fight, probably only lethal in an advantageous circumstance.
No one really took a pure caster out for a spin, so I don't have a lot to gauge on that front.
Initiative is incredibly important in this rules context. The Rage Drake could have 1 shot the KotW on the charge, had it not missed 1 attack. Even the Quez could just potentially end one of the party members in its natural setting via Snatch/grapple or Snatch/drop.
Ultimately, I'm pretty pleased with the format. Decision paralysis was pretty low, especially for brand new characters. Characters felt like they had pretty good options to try during each fight, even if the Dybbuk had immaculate defenses. Aligned weapons are well within reach, so it was just a matter of not having the right tools or preparation. Combat was pretty fast, and encounters felt threatening.
Some CRs are just not quite right unless preferred territory or circumstances, but that's not much to worry about. Next time I get the opportunity to run some encounters, I'll have proper environments. Perhaps some CR10-12 attempts.