Regarding Kyurem-B being a ***** to just purely passive stall, nah. Every kind of stall suffers against Kyurem-B. The sheer appearance of Kyurem-B regardless of set when im playing stall makes me go "oh ****". I dont play purely passive stall, as I believe that doesnt work in this metagame at all. I play stall that uses stuff like offensive starmie as a spinner and scarf garchomp as a pivot. I use other stall teams that run Scarf Politoed instead of the defensive sets and a Kyurem of my own for my offensive pivot. I can tell you Kyurem-B will **** me up from the team reveal. You might say its different in practice, but lets go back to your marth vs rey WC example.
Marths team was extremely anti stall (mainly because of Victini and not Kyurem but ill get back to that in a bit). When I saw Kyurem in team preview my first thoughts were "damn i hope thats not cb or sub". I won that game because his Kyurem was, like I said in my post in the Keldeo thread, a set in order to deal with offensive teams, Scarf. Scarf Kyurem is a nobody vs stall, but hes potent as all hell vs offense and him + scarf keldeo single handedly can destroy an offense team.
lets go to the highest level ladder imo, frontier. ive watched over 100 games in frontier and played in around 45, and kyurem is an absolute terror. sub 3 attacks kyurem, is catching up, and that set takes advantage of the necessity of offense teams to have at least one defensive pivot (celebi, rotom, etc), getting in on them, netting a sub and proceeding to kill whatever comes in. and kyurem continues the rampage. then theres scarf kyurem, who ive seen demolish offense teams by itself simply by clicking outrage. remember offense doesnt have many options for *defensive pivots* and they usually prefer taking on keldeo so the pivots are lead to him instead, that means ferrothorn is unreliable but the sole reason i can see him being used as defensive pivot in offense would be kyurem. because otherwise kyurem simply destroys. and then theres new set being popularized which is LO draco meteor kyurem with hp fire. enjoy your scizor switching in on a sub, or your ferrothorn.
and thats vs offense, vs stall we all know how it goes if the kyurem is sub roost or choice band.
going back to my game vs marth. simply seeing kyuremb made me misplay a lot. i led with chansey, saw kyurem and went **** now i have to switch out because this thing can do 80% to chansey and chansey is my win condition. and excuse me everyone who didnt see the match but for those who did, remember the back and forth switching between chansey and skarmory and kyuremb and venusaur for him? the idea of cb kyurem caused that. if i had known it was scarf chansey would have beat it, but kyurem had the ability to make me misplay simply because i saw outrage on a protect and it was too risky for me to stay in. and again this was a more offensive oriented stall team and not purely passive.
kyurem needs a test