1. They are vey different. Those who arent aware of the large differences shouldn't comment as if they are they are subject experts. Telegram might be full of holes but it is a very different thing to Slack. (I use both for different things.)
2. I have my own issues with Telegram (the refusal to introduce sustainable funding, pushing of crypto currencies, possibly also marketing it as more than it is) but except that there's a lot to like about it and I think it is a decent starting point if someone with strong cryptographic skills wants to start with a decent client and go ahead to provide something better (either introduce e2e-encryption or improve, document and provide some way to verify the current solution.)
2. I have my own issues with Telegram (the refusal to introduce sustainable funding, pushing of crypto currencies, possibly also marketing it as more than it is) but except that there's a lot to like about it and I think it is a decent starting point if someone with strong cryptographic skills wants to start with a decent client and go ahead to provide something better (either introduce e2e-encryption or improve, document and provide some way to verify the current solution.)