My own success record for funded projects is 100%. When people pay me (and/or my team) for a specific project, we get it done. This would be worth bragging about, save this is how it should be.
A project is something like "we launch in 2 years, make it work" or "get a best-fit model to this data" or even "get our books into distribution". It's a task that-- no matter how hard it may be-- any professional should expect to succeed at. You wouldn't want an accountant who says "no, I couldn't figure out your taxes after all" or a house painter that says "we stopped at wall 3, it was just too hard." Most careers have a high success rate at the project level.
It's worth noting that projects do not mean discovery. A project is an investigation. A discovery is a particular form of unpredictable success. So a research project might be to use Hidden Markov Methods to better predict CMEs. 'Success' means the method was implemented, tested, and compared against existing methods. 'Discovery' would be the result that the method outperforms existing methods.
Going back to the analogies, the investigation is like asking your accountant to do your taxes, and discovery is if they manage to find a way to get you a large refund. The first is the agreed-upon task, the latter is the best of all possible results.
Things get harder to define at the strategic level. You can apply Goethe's 3 review criteria to life and work: what are you doing, did you do it well, and was it worth doing? Succeeding at a project is 'just' craftsmanship, item #2. Doing it well. What to do, generally, is mandated by the nature of the job, the work proposed when you wrote the grant, or the bidding of a higher-up. Worth doing? That's for history, or the marketplace, or the peer review committees to cover.
So when a capable scientist says they are about to embark on Project X, give them the benefit of the doubt that they'll succeed. The real question should always be "why" and "what's the best outcome that might happen?"
Alex, the Daytime Astronomer
See also The Satellite Diaries, the diary of building our personal satellite.