JUCE is really painful to use if you want to integrate it into anything modern. It's old, it's slow, and you really can't make it better. I really can't express how much distaste I have for JUCE after shipping multiple products built on it - it solves _one_ hard problem (wrapping AU, AAX, and VST3), other than that it is dog slow and can't be improved because the core devs don't accept outside contribution for no reason except their own hubris.
Any cross platform UI in C++ that isn't JUCE or Qt is highly welcome. Especially one that allows me to pull in external dependencies and use modern build systems, test frameworks, CI/CD, and other tooling.