“The Wikimedia Design Style Guide ensures a consistent look.” However, the current guideline of typography is directly contradicting the central idea of the style guide by recommending "relying on the operating system's default sans-serif typeface".
Why
By not having a platform-neutral font creat inconsistency across platforms, increasing the possibility for devices not rendering Unicode that is not in the font they use on their systems. Not having a platform-neutral font also create inconsistency across languages.
There are already problems as small as inconsistency of logo font in Japanese between mobile and desktop and as big as unable to predict which CJK Unicode is not been able to display on user's device, creating unnecessary editing restriction.
In short, typography is just as important as the rest of the visual style guide and we should achieve consistency by setting platform-neutral fonts to achieve great readability and accessibility.
Suggestion
I suggest using the Noto font family as this open font family supports more language than any other open font, which is perfect for the Wikimedia Project. By using the Noto font family, we can make sure the consistency of the font type between languages. By utilizing woff2 format, we should be able to deliver the majority of the font in the family in a file less than 100kb, which should have minimum server impact. Also, since it is now 2020 rather than 2013, all the challenges listed in ULS's Webfont introduction have now been conquered by the newer browsers and newer formant, woff2.
Conclusion
As all challenges that previously prevent ULS to enable Webfonts by default for WMF wikis is not resolved, current typeface guideline should be reconsidered. It is time for WMF to embrace their fundamental ideology in their Design Style Guide by setting the Noto font family as the default font in the guideline of typography and let ULS enable Webfonts by default for WMF wikis.