From my experience, about 20-30% of the packages are not working with swiftpm, either because they don't have a Package.swift file, or because it is not compatible with up-to-date developer tools. On many projects, I had to fork a few repositories just to add or fix the swiftpm integration... while their Pod integration has always been working well.

That is the cost of using third party libraries, hardly the fault of either SPM or Cocoapods.

I‘d say it’s 2% now, of the maintained ones? I haven’t seen such a project in a long while now. React Native is the only one I can think of.