Earlier quoted context omitted.
There used to be advantage to using global CDNs: you would only need to download each font file once. But now, browsers don’t share caches between origins for privacy reasons (“hmm, judging by how fast these ten resources loaded (and how slowly these other thirty resources loaded), you had these ten in your cache; and such-and-such a sensitive site just happens to load those ten resources and none of the rest…”), so…
The other advantage of a font CDN is updating.
I am aware of them updating fonts that they commissioned. But I know of multiple cases where Google Fonts has been serving versions of fonts that are five or more years out of date, sometimes even broken by Google Fonts. Crimson Text I can kinda understand them not fixing completely, because fixing it would have changed character positioning (thus breaking people’s careful alignment to work around the Google-Fonts-introduced bugs). But there have been other cases where that didn’t apply: metrics were the same, but they just wouldn’t update the font. Might have been PT Serif? Lato? I can’t remember, it’s years since I last cared about Google Fonts.
In short: updating the fonts is not all it’s cracked up to be; Google mostly just doesn’t, and half the time you actually wouldn’t want the font updates anyway—depends on the nature of the update.