The biggest holdout I can see is that users of older Macs don't have webP support on Safari. I have an old Mac Mini that my wife uses to read her email that is that old and confirmed that webP doesn't work. On that machine, WebP works just fine with Firefox.
I decided it was marginal and that I was going to support JPEG and WEBP in my current project because:
1. I'm not publishing a huge amount of content and the storage cost of the JPEG isn't a problem, and 2. I want the system to be adaptive to what image formats the client supports (looking forward to JPEG XL even though I'm disappointed with AVIF) I am building the facility in and might as well demonstrate it works with JPEG.