Unless it supports all three mobile platforms or has published a full-featured API (or protocol specs) from day 1, any proprietary communication app is a non-starter for me (Hangouts included).
How one is supposed to promote it over WhatsApp, Telegram, LINE, and the likes? Hey, this new cool app does almost the same thing, but it's not available to the majority of your contacts!
In the real world, popular apps may be walled gardens, but they are cross-platform. Whatsapp supports 7 mobile platforms, Skype supports 6 platforms (5 mobile), LINE supports 9 platforms (6 mobile), Viber supports 8 platforms (6 mobile), WeChat supports 12 platforms (9 mobile variants), Telegram supports 7 platforms (3 mobile), Kik supports 3 mobile platforms, even Google Hangouts supports 2 mobile platforms. How an iPhone-only app can be considered seriously?