People ask this when they are deciding whether to apply, or whether to spend a month practising. They want a number. There is no public official one. You will see 40 per cent, or 50 per cent, or “about half fail the first time”. Treat those as gossip with a grain of truth, not a prospectus.
What you can know without a number: plenty of capable adults do not get through on the first attempt. That is enough.
Hard in a specific way
It is not hard like a maths contest. Most of the tasks are obvious once explained. It is hard like being asked to do an obvious thing accurately, for a long time, in a format you have never seen, after another format you have never seen.
Vigilance is the usual difficulty. Not because the rule is deep, but because thirty minutes of almost nothing is a different sport from five minutes of almost nothing. Group Bourdon is the other: a simple rule in a dense field, scored on both speed and mess.
If you have never done either, the first sitting is partly a test of composure. Composure is trainable. That is the optimistic part.
Why walking in cold is expensive
Attempts are limited. There is usually a wait if you fail. So the first attempt is not a free rehearsal. It is one of a small number of shots.
Practice does not buy you the official items. It buys you the feeling of the wrapper: where your hands go, how a miss feels, how long a minute is when nothing happens. That is a large part of the difficulty. Strip that away and you are left with the actual skill, which you either have or can build.
A useful definition of ready
You are not ready when you feel confident. Confidence is cheap. You are closer to ready when a full-length vigilance run is merely unpleasant, not chaotic. When you can do a concentration page without inventing fours. When a rules passage does not empty out of your head the moment it is hidden.
RailFocus will give you a practice score out of 100. That is a training signal. It is not a pass, and it is not what the operator will print. Use it to see if you are building. Do not use it as a prophecy.
So: is it hard? Yes. Is it mysterious? No. Mysterious is what it feels like before you have sat the work. After that it is just work.