- It doesn't compress well. It's twice the size of HEVC. We have similarly-performing VP10 now, but WebP is based on VP8, which is a 2006 codec that has lost to a 2003 codec in the market.
- It's actually 2 formats under one name. WebP has VP8 and its own lossless thing separately. JPEG-XR, JPEG 2000 and FLIF can use the same algorithm for both lossy and lossless.
- The lossy variant (which is the only interesting one) does not support full-resolution color. You have to live with chroma subsampling which messes up anything with sharp saturated colors.
- It's 8 bit only. This is too little for wide gamut displays which are becoming more common.
- It doesn't support progressive decoding at all (misses out on awesome HTTP/2 optimization that gives illusion of all images loading instantly).
- The alpha channel can only use lossless encoding. It's a big problem for non-binary transparency (complex shadows, smoke). That's the major usecase for it on the web, and that's the least efficient variant of WebP.
- It suffers from generation loss waaay more than any format.