The argument regarding "source code" is misleading and not entirely relevant. It's worth noting that Telegram's backend is also closed source, yet its supporters often overlook this fact.
Decompiling and inspecting mobile apps is relatively simple, so if there were any issues with the WhatsApp client, they would likely have been uncovered already.
As for Telegram, its messages are stored in plain text on their servers, and it doesn't offer default end-to-end encryption. This means that if Russian secret services were to gain access to Telegram's backend, they could easily read all the messages.
Therefore, when using Telegram, it's important to be aware that its administrators have the ability to read all of your messages.
Decompiling and inspecting mobile apps is relatively simple, so if there were any issues with the WhatsApp client, they would likely have been uncovered already.
As for Telegram, its messages are stored in plain text on their servers, and it doesn't offer default end-to-end encryption. This means that if Russian secret services were to gain access to Telegram's backend, they could easily read all the messages.
Therefore, when using Telegram, it's important to be aware that its administrators have the ability to read all of your messages.