The main downside of custom web fonts isn’t the licensing but having to download those fonts to render the page. There are ways to optimize font loading [1] but it can still lead to delay in rendering or font swaps.

It looks like ET-Tufte has already considered and tried to mitigate this downside. [2]

[1]: https://web.dev/optimize-webfont-loading/

[2]: https://et-jekyll.netlify.app/et-jekyll-theme/#performance