I mean.. that goes against all the stories I've heard from past employees. They test every set and it's effect on standard, but there is only about a dozen or less people doing it. So, it's not that they 'don't' test, it's that they can only test the basic outline of deck ideas. Luckily, over the years they have gotten more and more pro level players into R&D, so their testing has gotten better. Small things sneak through, but they have gotten pretty good at realizing what is busted and creating enough tools to fight any given strategy as a failsafe, even if they don't think it'll be good.
I personally don't think devotion was a mistake. Yes we had a pretty dominant deck, but the only times we don't is when there's too strong of a rock-paper-scissors effect, but with 2-3 extra options. Meaning, one deck (rock) just almost always beats another (scissors), irrelevant of skill level, but also auto-loses to another deck out there, etc.. I hate those formats, cause it's generally less skill intensive and just picking the right deck each week.
During mono-black devotion's dominance, you could outplay people in the mirror, or play an anti-devotion deck. I won lots of local tournaments/IQs with B/w midrange, basically black devotion with blood baron/obzedat, during that time. It was effectively the same % as mono black was against the other decks, little better against control, but it crushed the pseudo mirror.
I also don't see how they 'missed' rally, it was purposefully designed in such a way that it's pretty hard to go infinite with it, but you can't really play it as a normal value spell either. It's very clearly designed to be used exactly as it is, but they probably didn't have the optimal version of the deck.
My point is, magic players bitch and complain, no matter what Wizards does. We complain about fetchland prices, they reprint fetches, then we bitch that they've made standard too expensive.. We complain about modern not being a pro tour format, despite Wizards knowing that if the Pros focus on it, they will break the format cause they don't print cards with Modern in mind. We also complain about cards spiking in price. So, we get a modern pro tour, and what happens? Eldrazi breaks the format and spikes a bunch of cards, and we bitch about that too.