Does anyone know how these compare with the HEIC format that it seems my iPhone now uses by default? Is that non-mainstream, or otherwise not fit for comparison?
I haven't follow the trend much nor seen a recent comparison, so take this with a grain of salt. WebP is based on VP8, which was considered to be competitor to H.264 (AVC). HEIC is based on H.265 (HEVC), which competes with VP9 and AV1. So I'd assume WebP is one generation behind HEIC. WebP was introduced 5 years before HEIC though.
So WebP is hardly new, and a couple of generations behind. Now the next hotness is AVIF (or maybe JPEG XL), so ironically, WebP is becoming usable and obsolete at the same time.