The flaw of Big Kiruv is ultimately the same one that has ravaged traditional Judaism for centuries. It is why we left in the first place. It is why today, most Jews are not Orthodox, and will never be Orthodox, despite Liberal Jewry’s rapid attrition, and the high birthrate of the frum. A difference between the normative Orthodox "ex-Os" and the "baal teshuvahs" (newly ultra-Orthodox) is that the latter are not brought up with this nonsense and do not have the same familial restraints, so their fallout is naturally much higher as the truth fights its way to the surface despite every possible appeal and exhortation. And even with the BTs (baal teshuvahs) that stay, many of their children and grandchildren return to the secularism and Liberal Judaism of their ancestors for this very reason. Once you have seen the light, it’s out there. There’s no submerging it forever.
The fatal flaw of Big Kiruv on one foot is: Kiruv recruits are told that they are learning how to serve God. But in the end, they are taught how to serve rabbis. All else is commentary about this inherent structural flaw of traditional and fundamentalist Judaism.