RNG + excessive gear requirements are really the only way to add difficulty to something that is effectively a math puzzle. Even your idea of using negative conditions instead of good ones is a form of RNG that would end up equally frustrating if they occurred as often as the positive conditions we currently have.

The important thing to remember is that not achieving a 100% success rate is not failure. SE doesn't intend for us to have 100% success with them at this point.

Right now, Rapid Synth success seems to make or break the craft for me. I wish we were allowed to use mods to do things like track action success/failure because I definitely feel like my Rapid Synth success rate is closer to 30% than 50% but I don't want to have to manually go back through my chat log to count.