This is really a matter of preference. I do not use Flask-Admin on my projects, I have always preferred to design my own admin pages than being forced to accept the choices that are imposed by these big extensions. This allows me to have a consistent UI across admin and non-admin pages.

One thing that people often complain is that it is hard and/or tedious to build tables with data, which is one of the building blocks most admin pages need. Last year I wrote a blog article where I show how I build tables in my Flask apps: https://blog.miguelgrinberg.com/post/beautiful-interactive-t....

I think that Flask has many great extensions, but in my opinion, installing flask-admin is just a tacit admission that you really wanted to use Django instead.