I've never heard of JUCE before, but from what I understand JUCE is more like the web platform itself (juce::Component is like DOM or canvas elements), while XM

I've never heard of JUCE before, but from what I understand JUCE is more like the web platform itself (juce::Component is like DOM or canvas elements), while XMLUI is more appropriately compared to declarative UI systems built on top of JUCE (GUI Magic, JIVE, or VITRO). Declarative and imperative UI approaches aren't mutually exclusive, and it's common for software to leverage both (e.g. SwiftUI and UIKit).