Whatsapp also makes design decisions that I would call 'engineer-easy'. Their iOS app is pretty much all default system widgets and behavior for example, which

Whatsapp also makes design decisions that I would call 'engineer-easy'. Their iOS app is pretty much all default system widgets and behavior for example, which is significantly less code to maintain than something that was customized heavily. On the server side they don't retain conversations, have multi-session usage (many clients, one user), etc which simplifies what is required. I haven't used whatsapp in a while so things might of changed, but before they were acquired by Facebook, that was definitely their direction.