Except compatibility, but the biggest gap is browser support, which is in the process of getting closed. Chrome has shipped JXL support behind a flag. Firefox are in the process of implementing support.
In Chrome you can enable JXL from here:
chrome://flags/#enable-jxl-image-format
In Chrome you can enable JXL from here: chrome://flags/#enable-jxl-image-format
You can track Firefox progress from here: https://bugzilla.mozilla.org/show_bug.cgi?id=1539075