I know stall isn't that great and all now (anyone saying it's dead/sucks has not played a true BW2 stall team yet) but this statement is nowhere near true. If your stall team cannot handle CM Keld, then it doesn't even matter what team you're using then because you are THAT BAD of a player. Toxic Tenta, Amoonguss, Jellicent, etc. Stall should be able to handle Keldeo pretty well...
while i agree that CM keldeo should really not be able to rip down a well made team, it's not easy to make a well made team on the defensive side in this era... which is the real problem. we have counters to almost everything (even in this era, very very few threats can claim to be uncounterable - even hydreigon gets countered by physically defensive chansey, and jirachi can handle tornadus-T, and heatran can handle genesect, etc. gotta say the closest i've come to uncounterable in my experience is standard fblast/eq/outrage salamence), but not enough of those counters are counters to multiple things, and not enough of those counters fill important team roles to form team synergy. with only six mons, stall can't fit enough counters to stay alive, long enough to win, against most teams. it's not an era where smart play will let you outstall even the mightiest powerhouses, because with only six mons, you will be missing a counter to one of those mighty powerhouses.
at least i can say this is true for weatherless stall; trying to play weatherless stall right now is basically gambling on ladder for someone to run the thing that beats you (cause oh boy, there's something in the top 40 that beats you in this era). it's something i have definitely tried; it always falls apart. it's not impossible, and you'll be able to hold your own against many teams, but there will be SOMETHING that beats you, and it won't be something rare and niche either.
i mean sure every team has its weakness, but i find without fail that weatherless stall is always weak to at least one common threat, which is a lot more serious of a problem than "i have a fatal weakness but it's somewhere down in RU". if we look at lavos sun for example, stuff like shed shell heatran or special RP landorus was quite rare prior to that team (and similar ones) becoming popular. at least in some part, this archetypal sun team was responsible for the popularization of such mons. not so on weatherless stall. not running physdef skarmory? oh sup you probably lose to mamoswine. not running chansey? oh sup you probably lose to hydreigon. these threats are not exactly "rare". they're solid OU and you WILL meet them, and when you do, you lose. period. one wall goes down cause you can't outpredict and suddenly your whole team falls apart because stall can't function if a key wall is down. i used to be running sdef skarmory because i had no room for jirachi (already had two steels and tentacruel, how much ground weakness do you want on a single team?) and every time i met a mamoswine i lost. is that what you call shitty teambuilding? of course it is, i'm a shit player! but when i switched to physdef skarm i started having more difficulty with tornadus-T. there is only so much room to optimize within 6 mons. with the artificial limitation of weatherless stall in place, i doubt i could improve my team beyond a certain point no matter how good of a player i was.
i imagine weather semistall/balance is not quite so polarizing. haven't had much play experience with those to be honest. anyway i don't know what will fix the problem nor am i experienced enough to know (some would go so far as to say that there is no problem in this regard - i doubt i agree with that), but that's my experience trying to play weatherless stall - it's always a fun one, but not always in a good way. make of that what you will
unrelated side note: holy crap i just found the increase/decrease size button on the edit box, where has this been all my life