Back when I got a Pixel 6 to try out, I was pleasantly surprised with the initial Android set up process. Especially as there was a way to transfer WhatsApp data (chats, history, media, etc.) from an existing iPhone.
However, when I decided to switch back to my iPhone being my primary device, I discovered that the reverse process just wasn’t possible. Which wasn’t exactly the end of the world, but as I’d only switched for a few weeks, but would certainly be a much bigger friction point if there was a lot more data involved.
This process will sign you out on your old device. Until you delete the app, of course. But it sure would be nice if WhatsApp allowed you to be signed in on multiple devices at the same time. It would make transferring between platforms even easier.