The impression of a walled garden hardens for me when it comes to interoperability. For instance, instead of providing gateways to standards such as XMPP or IRC, as a developer I am confronted with an absurdely complex API. It is rich and versatile, but impossible to implement a client in a couple of hours (not talking about their bot API, this is not a full client).

Even worse, one can only participate in their network with a mobile phone number. In my country, land line numbers do not even work.