Some sites like Instagram don't allow access without logging in, and so can't be archived. It might be different if the people behind archive.is create an account there, like they have for twitter, but they don't at the moment.

Twitter can be archived, but they've disabled various links for archive.is. Like if you archive a quote tweet, you can see the quoted tweet, but the links to the original tweet or tweet author are disabled. This was something Twitter did in the last year. Nitter doesn't have this problem. Twitter also heavily reduces the number of tweets that can be seen on a profile page, and how many (if any) replies are shown, when archived.

Some sites like Instagram don't allow access without logging in, and so can't be archived. It might be different if the people behind archive.is create an account there, like they have for twitter, but they don't at the moment.

Twitter can be archived, but they've disabled various links for archive.is. Like if you archive a quote tweet, you can see the quoted tweet, but the links to the original tweet or tweet author are disabled. This was something Twitter did in the last year. Nitter doesn't have this problem.