By the latter I mean that with legacy media - despite the political slant (either way) of the publication as a whole, you generally have access to a multitude of voices.
With the Substack model that isn't the case - unless you limit yourself to the free options, or are wealthy enough to subscribe to as many writers as you like.
I suppose there are Substack amalgamations out there, grouping writers on a common outlet, but those who have managed to gather enough of a living striking out on their own are probably not going to worry too much about whether they are also available as part of a more economically accessible and diverse collection of voices using a different subscription model that collates multiple writers for a single cost.
Ultimately that means that access to a truly wide range of authoritative voices is potentially limited to the wealthy.