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
There is much better than JPEG, however, because still images are not really a problem in terms of bandwidth and storage, we just use bigger JPEGs if we need more quality. The extra complexity and breaking standards is not worth it.
This is different for video, as video uses a whole lot more bandwidth and storage, it means we are more ready to accept newer standards.
That's where webp comes from, the idea is that images are like single frame videos and that we could use a video codec (webm/VP8) for still images, and it will be more efficient than JPEG.
That's also the reason why JPEG-XL is taking so long to be adopted. Because efficient video codecs are important, browsers want to support webm, and they get webp almost for free. JPEG-XL is an entirely new format just for still images, it is complex and unlike with video, there is no strong need for a better still image format.
IMO most of JPEG-XL's value is in the feature set. Having a format that can do transparency, proper HDR, and is efficient for everything from icons to pixel art to photographs is a really strong value prop. JPEG, Webp, and AVIF all have some of these, but none have all. (AVIF is the closest, but as a video codec it still has a pretty significant overhead for small images like icons).
