1. When the precise time of birth is uncertain, astrologers sooner or later come to this stage, they begin to study methods to adjust the time of birth. But the courses, lectures and schools teaching human design skip astrology as a compulsory subject. Neither is required from students to have the necessary astrological competence. I do not know and have no logical explanation why they  have dismissed it from the curriculum though. Probably, the reason is that in order to master the whole astrological system, and on top of that to be able to use it practically, takes many years, and this turns into a huge barrier. And if the practitioner (a non-astrologer) of human design is faced with imprecise time of birth, the only thing he/she can do is to send you to an astrologer.
2.  Imagine that at some point you have no access to a computer program to calculate the rave chart required for a human design analysis. If the practitioner doesn't have the skills to work with ephemeris and does not know how to calculate the astrological horoscope, he/she has no choice – they will need to turn to an astrologer.
3. Human design analyses are mostly basic. The estimates made ​​have nothing to do with the predictions made by astrologers. You can define the time in which an important "fill" is formed out of a slow planet transit, but you cannot get a simple explanation of how and in what way you will experience it.
Questions of the type below cannot be answered:
When or if I will find a job.
When or if I will get married?
When or if I will have children?
And we should be honest to ourselves - these are issues which are almost always the most important reason why people turn to esoteric knowledge and predictions as a whole.
4. When analysing partner relations, what is considered is only the base, the foundation. Human design cannot trace evolution in time without an astrological apparatus and it cannot answer whether certain relationships will last in the future. The key parameter for such a response is the time when the people analysed have met, which again requires astrological predictive methodologies.
Now, let's discuss the human design pros.
I wouldn't like you to be left with the wrong impression. In the beginning, I mentioned that I was deeply impressed by the information that can be extracted from people, very easily and quickly, at first glance. Not that this can't be done by means of astrological methods, but there can be some missing pieces now and then, which leads to certain ambiguity or even uncertainty.