no versions for systems other than Android and iOS

requires phone number (common argument)

Ok, those are legitimate complaints, and I suspect they're related too. It would be nice to have a web client.

hard to integrate bots, notifications and automatic services for the future use

Personally, I'd say that's a feature.

Signal is super giga great, the cons list is short, but if we want everyone to use something it has to be an universal protocol, not one app.

To be fair, signal is an open source protocol that anyone is free to implement. Signal protocol