Push notifications are a major one. You can't have PWA Twitter/Slack/WhatsApp for example on iOS because it won't be able to receive messages when it's not the active app.
I consider the lack of notifications a feature and use the Twitter PWA.
Is the "Twitter PWA" something other than "www daht twitter dot calm" in a web browser?
The same. But you can add to home screen, so it feels like an app, kind of. It's pretty bad, but honestly I want social media apps to be bad to mitigate the addictiveness they build in.
notifications are an opt in feature, just like they are for native apps. If you don't want them you turn them off or don't opt in in the first place. Me, I want them on for private messages only. If twitter didn't allow that i'd turn them off. But apps like slack or at least apps like whatsapp etc would be kind of useless without push notifications.