Whatsapp being phone only is what kills it for me.
It's not a bug, it's a feature. It's what allows it to take the place of SMS. If your contact is on WhatsApp and you message them, you know they will get your message on their phone. If you send a message to someone on facebook messenger, they might not get it until they log in to facebook on their computer next week, so unless you know their habits you have to send an SMS if it needs to be timely.