Substack faces a lot of possible problems, imho: does not scale that well, limited audience size, writer burnout https://greyenlightenment.com/2022/06/22/substack-worth-1-bi...
I think what substack needs is better content/writer discovery. There seems to be no good way to find related substack blogs adjacent to the ones I already follow. Medium .com at least got that part right. I can go to the home page of Medium and find huge assortment of articles that interest me. That's not the same with Substack.
Also, the the pop-up prompt to subscribe on mobile is annoying and does not go away. It should be disabled via cookies if you decline to subscribe.