"Reliable" is a relative term for WA. Everybody I know has a random delay in messages, the browser "thing" is mostly not usable, no desktop app, etc. You can not use WA for productivity.
basically they shot themselves in the foot on the mobile market with an half assed implementation, and even today the notification system on skype mobile apps is dumb, especially how they handles balloons (the only app that could do worse on notification balloons is facebook)
whatsapp may not be a cornerstone of reliability, but it has an incredibly frictionless mobile user experience.