"Not all messages and chats are synced to linked devices from your phone. WhatsApp Desktop syncs more message history than WhatsApp Web. To see or search your full history, check your phone."
In whatsapp, there is a primary source (your phone) and you can "link" other devices with the primary source. This is different from how telegram handles it. In telegram you can use sms login to use the client from multiple clients, within the same smartphone, or different devices. With whatsapp, you can only have one sms login device, which would be primary device. If you use sms login with another device, you wont be able to use the previous device. To overcome this, it has a feature which can allow you to link your primary client with other clients but it is janky and does not have all the features. For matrix on the other hand, the encryption keys are tied to the account.
AFAIK, whatsapp is still dependent on your mobile being the primary source. It has "link a device" feature, which has some of the features but cannot be used as an independent client. For example if you're using the web version and want to see some old chats, you can't unless you use it from your mobile. WhatsApp's encryption keys are tied to the device, not the account. In Matrix, encryption keys are tied to the user's account, not the device, and thus it wont be an issue.
Telegram can be used on multiple devices simultaneously. Syncing the private key for each chat across multiple devices can be an issue. That's why secret chats are only available on the device they were created on, and having encryption on by default would not make sense.
because telegram has group chats and channels, which are also accessible cross platform, it doesn't make sense to turn on e2e by default. secret chat feature is encrypted and thus only available on the device you start it from.
From the blog:
"if your primary device is inactive for a long period, we automatically log you out of all companion devices."
There is one primary device, and multiple linked companion devices. This is different from telegram where there is no primary device.
Also see : https://faq.whatsapp.com/653480766448040
"Not all messages and chats are synced to linked devices from your phone. WhatsApp Desktop syncs more message history than WhatsApp Web. To see or search your full history, check your phone."