HEIC does not open in most browsers, on many Windows PCs or in a lot of upload forms. QOI is lossless and extremely quick to read and write. HEIC is at home in saving storage on Apple devices; QOI is the usual choice for game textures and pipelines that need very fast encoding. QOI cannot bring back detail that HEIC compression already removed; it simply stores the result without further loss, in a larger file.
The defaults suit most files. Under Options you can resize the result.
Most browsers other than Safari cannot read HEIC themselves, so a HEIC decoder of under 1 MB is downloaded the first time and kept in the browser cache. QOI stores every pixel and the alpha channel exactly; the encoder is a small WebAssembly module that downloads the first time and is cached.