The newest format is also the least compatible one
AVIF is genuinely impressive from a pure compression standpoint — it typically beats both JPEG and WebP at the same visual quality, which is why some of the biggest websites have started serving it by default. But being the newest format cuts both ways: the same recency that gives it excellent compression also means a lot of software simply hasn't added support yet.
This shows up in very ordinary situations — you save an image from a website, and it just doesn't open in an older editor, won't attach to a form that validates file types strictly, or displays as a broken icon in software that hasn't been updated recently.
What your browser can do that other software can't (yet)
Current versions of Chrome, Firefox, Edge, and Safari can all decode AVIF natively, which is specifically what makes browser-based conversion possible without installing anything — the browser reads the AVIF file the same way it would to just display it on a page, then re-encodes what it read as a JPEG. Software outside the browser doesn't get that same built-in decoder for free, which is the entire reason this compatibility gap exists in the first place.
This is a temporary situation in the bigger picture — format support tends to spread over a few years as more software gets updated — but for right now, JPEG remains the safe universal choice when you need an image to open reliably absolutely everywhere.
Watch for transparency the same way you would with WebP
AVIF supports transparent backgrounds, and JPEG flatly doesn't. If your AVIF image has a transparent area, converting to JPG needs to fill that area with something — a good converter lets you pick that fill color rather than defaulting to something that clashes with wherever you're about to use the image.
Is it worth converting your whole library, or just the one file you need?
For a single downloaded image you need to use somewhere specific, converting is the obvious move. If you're producing a lot of AVIF images yourself (say, exporting from a modern editor) and running into compatibility problems repeatedly, it might be worth exporting as JPEG directly at the source for anything destined for wider distribution, and reserving AVIF for contexts where you know the audience's software supports it — your own website, for instance.