You know, as someone stuck using XHTML within the Java ecosystem for web apps, i'd say that it's reasonably easy and nice to work with. However, i'd also like to suggest that custom tags, at least when implemented in the ways that are popular in those particular technologies (JSF/PrimeFaces) are bad both from a usability and maintainability perspective - it took many years for React to come around and make components easy to create and utilize. XHTML with the semantics of React would be way better.
That said, i'm also stuck in the middle of migrating older versions of frameworks (Spring) to newer ones (Spring Boot) and it's proving to be extremely cumbersome and feels like it'll be impossible to get right without spending multiple months on the migration. Most of the older technologies are considered "dead" nowadays for a reason, though at the same time i agree that they can hold a certain wisdom.