What are the specific cases that justify Spring selection over Spring Boot?
You can found several cons on Spring boot over Spring, mainly missing learning steps in Spring and support and if using monolithic application
If you are new to Spring and want to learn how the dependency injection, AOP programming, and proxies work, starting with Spring Boot is not a good choice. Spring Boot hides the most of these details from you.
If you are not familiarized with other projects of the Spring ecosystem (Spring Integration, Spring AMQP, Spring Security, etc), using them from Spring Boot will make you miss a lot of concepts
In a large and monolithic based applications, I wouldn’t encourage you to use Spring Boot
Also can increase deployment size
Spring boot may unnecessarily increase the deployment binary size with unused dependencies.