use of overlays for core user journeys. good or bad (ELI 5)

firstly, thanks for reading!

I am involved with the design of a finanial web app that is primarily tablet/large screen based (however will have to work on mobile). I am not responsible for the actual design, which is being created by a 3rd party, but I am responsible for auditing the usability/UX aspects of it.

The 3rd party has implemented a modal approach to delivering the core onboarding journey (over the dashboard) - now, I think this is not an, ahem, optimal solution and I am aware that modals have issues. This article (What research is there suggesting modal dialogs are disruptive?) explains some of it.

This is an example of what they are designing

I am hoping that you can help me to explain to non technical management why use of modals for the main journey is not a good idea. Simple examples would be great. :)