Teg was able to see no-ships at the end of Chapterhouse. Does that mean he could see prescience resistant people as well? Does that mean Leto II's breeding program did not do what it was supposed to do? Or were there other implications on the bigger picture?
Leto II's breeding program achieved Siona -- a biological protection against prescient seekers -- and his enforced tranquility forced Ix to achieve no-rooms and then no-ships -- mechanical protection against such seekers. In this regard, Leto II did succeed, twice over, in creating the conditions he most desired, wherein all of humanity could never be trapped by a single enemy and exterminated.
Nothing, however, is permanent, and Leto II deliberately did not seek for perfect knowledge of all possible futures. All he sought was a path that gave humanity the best chance for survival. As such, he left the universe open to surprises, including the possibility that eventually, someone would develop, deliberately or accidentally, the ability to nullify either the no-room effect or the "Proof of Siona".
Even so, Leto II clearly believed (and I believe said as much in God Emperor of Dune) that the Scattering would so spread humanity across the universe (and possibly other universes) that it would be impossible to find them all an exterminate them, even if they could be seen. The no-ships and Siona were not intended to be a "forever" solution, because, as Leto II himself says (this one I'm sure of, having just reread this passage recently), against the backdrop of infinity any "long term planning" is still "short term" and ultimately doomed to fail! They were intended, rather, to change humanity's outlook to want always to seek out new places to be, and to give them a breathing space to find enough new places that, when at last something came along that defeated both the biological and the mechanical ways to hide for searchers, there would simply be too many people to ever get them all.
It's implied in Chapterhouse: Dune that he might have miscalculated, but it's difficult to be certain. Clearly, at least one segment of the Scattering, and the Old Empire, were in danger, but it's never really clear if all humanity was!