Web font load failure rate.

Do you mean the percentage of site visitors whose browsers don't even support web fonts? Less than 1% these days, assuming fairly standard approach to web fonts (and that's without serving SVG fonts).

If you mean the percentage of site visitors who don't see the fonts for any reason, including failures of service, or the site or service requiring JavaScript, that gets higher. Requiring JavaScript probably stops 1–2% of actual users all by itself. Failures will of course vary by service, and are generally quite infrequent.