The moment you hook into those systems, you're segmenting your userbase. If you rely on iMessage for transport, you can only talk to Apple users. The situation is similar with Hangouts and WhatsApp - while you're not limited to one device manufacturer, there's no official API, and any attempt to do so is a hack that could break at any time with no warning.

Telegram is slightly better in that it at least has a client API, but you're still locked to a single provider (the official Telegram server).