Well, I think there is a valuable set of use cases where admin themes and personalized backend can play a great role. It does not need to be necessarily a reimplementation of the backend.

We worked developing custom solutions for clients for years and we definitely see a much better response with a custom backend experience. And most of the time, it was just a css theme applied to the WordPress backend.

WordPress makes it dead simple to build a MVP to your SaaS project. In that case, even if you don't want to/or don't have time to create a custom backend experience from scratch, it can't hurt to personalize even a little the back end. WordPress is widely known and even the most non-tech-savvy potential clients can indentify that it is in fact using WordPress and that can hurt the business as a whole.

In fact, we took our most popular custom-built styling solution and released it to everyone. It is a completely redesign of the WP Admin, using Google's Material Design. We're calling it Material WP, you guys can check it out by clicking here.