"Also doesn't support multiple device."

Not strictly true. You can log in a second device (e.g. web browser, tablet client, desktop client) by scanning a QR code with your primary device.

My iPad is my secondary device. Not all messages are synced, but I can pick up my iPad to start or continue a conversation or do other stuff, without doing anything special. The full conversation history is only on my phone, though. The iPad only has the messages sent/received whilst I was actively using it.

Agreed. But their audiences are folks never need/own a desktop/laptop in the past. In China and many South-Asian countries, ownership of smartphones are higher than PC.