When you are not afraid of jab/cross, you will have more courage, everything will then move toward your advantage.
I agree. Once you are confident of handling this basic common attack, it frees you up to "concentrate" (I put this in quotes as it's really the opposite, but that's another topic) on what
you want to throw and further control the action. Being hesitant in a fight is a sure way to lose. But to get confidence takes constant drilling, and even getting hit once in a while to learn that taking a hit is survivable and should be expected in a real fight. The main thing is to always have an offensive spirit, even in defense.
A good technique is a technique that should work with high percentage.
True. For this to occur, speed and timing are needed as you state. A couple of other factors in my definition of a good technique is that it should be as simple as possible, efficient, minimize your exposure to counter, and leave you in position to advance your attack. Of course, each of these things is a whole subject in itself. That's why I agreed with Flying Crane that there's more to a basic technique than simple motion and not easy for a beginner to execute it effectively against resistance on day, week or even month number 1. Besides, what's the hurry?
To follow this a little further: Karate, as a business that must often cater to the Western scourge of "immediate gratification," an instructor has to make the new student feel that he has achieved the ability to protect himself from almost day one. The student learns to defend a head punch and wrist grab and goes off happy he is now a karate man. He feels good about it and so is likely to return to class and get more of that feeling. Naturally, this is delusional as the student will likely be helpless from such attacks done with ferocity or resistance.
If the instructor is
lucky enough not to need the "average" student, he can tell a new student not to expect to do anything meaningful with what he's learning for a couple of months, that it may even be a year before he can be confident of reliably defending against a real attack. If the instructor is not that lucky, the prospect may leave and look for a school that can sprinkle some fairy dust and turn him into an instant ninja.
So the instructor is in a dilemma - Setting a time frame that contradicts the student's expectation of easily won skill, realistically setting the expectation of being prepared for the long game, or building the student's motivation and (false) confidence of gaining quick skill from this great school. These have to be carefully balanced to gain and retain students, while maintaining some honest integrity.
Maybe the answer is to "Fake it till you make it," and hope the student doesn't get into a fight in the meantime.

Seriously, I do believe there is a balanced approach including giving the student small wins along the way on the basics - "Good stance! See how that keeps me from pushing you over?" rather than, "See how you can block any punch in a fight?" I think this is the way to go.