Spring Boot certainly makes it very easy to get started, and to grow your system in the way that Spring envisions. For getting started, there's even the Initializr:

https://start.spring.io/

However, as soon as you want to go off the beaten path (eg to have two databases), Spring starts to become an obstacle rather than an assistance. In this respect, it's similar to Rails or any other large, opinionated framework.

I love spring boot. I use two databases and solr. It's easy. And fun.