Whenever I want to use pictures, to host/post on forums like this, or submit medical reimbursement, etc etc, I end up having to change Safari's Developer menu to force download as JPG out of iCloud. This is not the end of the world, but somewhat tiresome, especially as I often forget the first time. Apparently I can set iPhone to "most compatible" which would then take the pictures as JPEG instead of HEIC. What are the downsides to that?
Basically, greater compatibility is the main thing in JPEG's favor, while HEIC beats out JPEG in most other categories. HEIC images takes up less storage space on your devices due to their advanced compression method, they are higher quality than JPEG, and retain their quality better even through multiple edits, unlike JPEG, which degrades quickly upon editing/saving over and over again. Adobe goes into a little more detail about the differences here: HEIC vs. JPEG: Which is better? | Adobe
If you change to JPEG, your videos will no longer have HDR. If you need to quickly change a HEIC photo to JPEG, just take a screenshot of the image. Then edit/post that (it will be JPEG).
