I calculated the time, and I found that it is not only faster, but 100% safe, if you can complete the craft in a single Careful Synthesis (or CS II), to just spam "Synthesize" and then "Careful Synthesis", "Synthesize", "Careful Synthesis" etc.
There's this long pause between synths with Auto-Synth, and you got the fail chance (something that is impossible if you do it with CS) which is annoying.
IMO, Auto-crafting is only good if you wanna make a sandwich and don't care about failures.