HEIC is what iPhones save by default, and it is genuinely good: roughly half the size of JPG at similar quality. The problem is everything else. Windows needs an add-on, many websites reject the upload outright, and a lot of software simply cannot open it. Converting to JPG gives you a file that works everywhere, at the cost of some file size.
HEIC is not natively decodable in most browsers, so the tool loads a dedicated decoder and does the work in JavaScript on your device. That is slower than the built-in decoders used for PNG or JPG. There is a 30 second timeout to prevent a large file from hanging the page.
Live Photos pair a still image with a short video, and edited or unusual exports can use HEIC features the decoder does not handle. Try the unedited original from your camera roll. Ordinary still HEIC photos convert reliably.
No, and this matters more than usual here, because personal photos are exactly the kind of file you should not be handing to a random website. The decoder runs in your browser and the image never leaves your device.
Usually yes, often close to double. That is the trade you are making: HEIC is the more efficient format, and JPG is the more compatible one.