The criteria above are not absolute of course, but they do matter. What we are looking for technically is that your shoulder position is rock solid in the basics above. The shoulder stays in an elevated position and is unaffected by leg movements or by longer holds. When doing presses, the shoulders stay as stacked as possible and the back rounds in the lower ranges.
All of the above signifies a shoulder placement that is ready to be challenged in much more complex world of the one-arm. If your shoulder are not able to stay stacked while you do regular two arm work they will very likely give in very quickly as soon as you begin transferring the weight to two arm. Not only does the arm need to carry significantly more weight but is also challenged heavily by much more complex balance issues in the lateral and rotational directions.
On top of this we also have the flexibility aspect. Having a good middle split and pancake will be the direct positions that will help you the most to learn the one-arm. The straddle one-arm is by far the easiest position and the one that makes the most sense to begin learning. The wider the straddle, the easier of a time you will have with it. The width of your straddle and depth of pancake will impact how low your centre of mass will be as well as allowing you to control rotation better when learning.
Remember that prerequisites we have set here are meant to be generalized to set realistic expectations and provide a guideline for the average practitioner. Then of course we have the outliers. Those who are able to begin training it without “fulfilling” classic criteria as those above. There are rather few training forms that give a meaningful carryover to handbalancing, but people with a lot of acrobatic experience can sometimes have beneficial spatial awareness. People from disciplines such as breaking, capoeira, other circus disciplines, etc might be able to access the one-arm balancing sensation faster than others.
However, we are looking for a consequent and replicable approach that we know can be applied again and again and to do so we need prerequisites, methodology and a technique. Learning the one-arm handstand takes a lot of time even with the best of preparation. From our experience, the average person spends between 1.5 to 3 years developing their first straddle one-arm even with a pretty good level of preparation.
The process can be long and often frustrating. There will be ups and downs in it and injuries might make it even longer. In this way its important to know what you are in for and to enjoy the process and the practice itself even when the result you are looking for is far in the future. A better question to ask for many would be if you are currently enjoying your handbalancing practice enough to keep doing it for years.