Some resources on PMI validity plus comparison of different frameworks.

https://www.cio.com.au/article/402347/pmbok_vs_prince2_vs_agile_project_management/

No unified model in project management

http://www.engineersjournal.ie/2015/08/11/critique-project-management-professional-discipline-private-public-non-profit-sectors/

Why the PMBOK is not a methodology

https://projectmanager.com.au/pmbok-not-methodology/