That said, I'm still on the side of betting with history. In most organizations, the deadline is the first date that nobody can (yet) disprove. When a deadline depends on problems not happening on this project that historically have been common, I think it is safe to bet that history will repeat itself.
This goes doubly for the SLS. Which is more ambitious than past launch systems, and is being built so long after the last new launch system was designed by the companies involved that there is little institutional knowledge left about how to do it. (Furthermore building with competing companies contracting for pieces that need to integrate just sounds like a recipe for expensive overruns to me.)
As opposed to the BFR. Which is being designed by a company with more recent experience of how to build new launch systems than the rest of the planet put together.