You give up server redirects and cookie auth. (well you don't lose cookie auth, but you need extra steps for it to prevent CSRF)

You can just have the server return a json object instructing the frontend to navigate, accomplishing server redirects.

Javascript can do browser redirects, but those are not actual 301 HTTP redirects.

Yes, what I'm saying is that you can have the backend return e.g. "{ "response": 301, "redirectUrl": "https://example.com" }", and then have the frontend handle that properly, and you get the functional equivalent.

I'm aware that it is different under the hood.