Completing a PHD is usually a very hard task and involves a lot of sacrifice. Moreover, most of the time it is not useful in your career (unless it is a pre-requisite of your job, i.e. research scientist). So what is the dark side of completing a PHD ?
1) risk of developing health issues (physical and mental) especially stress related to the culture of "publish or perish" (see nature.com/articles/d41586-021-01751-z)
2) risk of destroying one social and marital life (who can enjoy a stressed partner working 60-70 hours a week and constantly talking about its thesis subject ?)
3) risk of developing a tunnel vision oriented towards the thesis subject (PHD students are trained to become experts in their own field) and often the interdisciplinarity is overlooked which limits their capacity to solve real complex problems (which often needs a multidisciplinary approach)
4) risk of become overcritical (using as a pretext the scientific method and fair criticism to the point of destroying colleagues)
5) Become heavily biased (tunnel vision, school of thought, vision inherited from colleagues or supervisors, etc.)
6) Missing opportunity of life (developing other skills such as music, sports, etc.)
Despite an obvious dark side, I think it is worth to complete PHD, however, one has to be very careful in not falling into the traps described above.
If anyone feels to add up on this discussion, please do so.