What WebP offers
WebP supports both lossy and lossless encoding, as well as transparency. Which properties the output actually has depends on the encoder and its settings.
Do not read a high quality percentage as proof of lossless output. Filecozy’s quality-controlled conversion can change colour detail, so inspect the result.
When JPG is the simpler choice
Use JPG when an upload form, editor, or publishing system requires it. Check the destination instead of assuming that every application accepts WebP.
If you need transparency, JPG will not retain it. Converting WebP to JPG fills transparent areas with a background colour and can introduce further lossy changes.
Compare before replacing your originals
Try a few representative photos and graphics. Compare bytes, fine detail, text, and transparent edges. A saving on one image does not predict the saving on all of them.
Use the output in its intended context before converting an entire collection. Keep originals so that you can make another version without repeatedly recompressing the same file.