Theoretically Social Security is "pay as you go". In fact it is putting away money which is "invested" in US treasuries. The result is that we can run up debt faster without having to convince anyone that buying our bonds is a good idea.
Which means that when time comes to pay out, the US government has to pay back that debt to Social Security, which the US government shows no signs that it will be able to do.
Furthermore, even if you believe that these "investments" are good, there is a massive shortfall in Social Security looming.
Back when Paul Rubin was head of the Treasury, he and Allan Greenspan agreed that, based on then-current figures, the optimal implementation of Bush's plan was to opt-out people below 37 into the new approach (they could expect more) and then government would have to fund the 1 trillion projected shortfall in Social Security.
I strongly sense that Bush does not wish to find that trillion dollars, and he doesn't like the idea of leaving current Social Security as a government program which lasts for decades more (until the people who are now 37 or older die off). Furthermore he is well aware that he can just supply an optimistic estimate for growth between now and when people retire, come up with a plan that will only work if his estimate holds, and then it is someone else's problem when it breaks.
Those beliefs are why I do not trust Bush's planning.
Cheers,
Ben