That Telegram has handed their keys to the Russian government? Difficult, but unnecessary.
That Telegram was deliberately designed in a manner which enables its owners to easily hand over the keys to the government? Easy to prove, Signal and Whatsapp do not share the same surveillance features.
Who pays for it now that it is free?
I for one refuse to think that Facebook bought it for a number of billion USD and keep operating and improving it for no good reason.
We also know, based on what they had to tell EU and based on their new EULA - that they tried to introduce by default - that they wanted to harvest metadata from it.
Attacking Telegram while simultaneously praising WhatsApp seems really strange to me.
As for Signal, that's another story. They have their own issues but I personally believe it is better than both Telegram and WhatsApp.
Look at them both from a purely technical perspective.
WhatsApp has made an effort to ensure that they're limited to only monetizing the metadata of the chats, Telegram by design does not share this limitation.
Even if you don't trust Whatsapp, you know that they'd have to push a backdoored app to everyone in order to intercept your chats. Telegram is backdoored by default.
None of the other messengers you mentioned are designed to work like this (not 100% sure about Viber), the ability of operators to read most of the conversations on the platform is very much unique to Telegram.
It's really not a coincidence that the Russian government is choosing to ban the worst "encrypted messenger" on the market.