Wagtail's authoring interface and Django's admin coexist. I believe the demo sets up the Wagtail interface at /admin and puts the standard admin at /django-admin. Presumably you can change those in your urls.py to whatever you like.
Two admin interfaces with different appearances and UI conventions isn't a terribly optimal user experience.
If Wagtail even attempted to smooth the differences between the two (i.e. a custom skin and some customizations to the default admin to make them more similar) then it might be a bearable compromise.
That alone makes you slightly 'an island apart' as I now face the challenge of either:
a) build my own CRUD for parts of the system that aren't directly related to the CMS (products, crm etc etc)
or b) rely on integration with your admin - which I'm guessing isn't as extensible or mature as Django's