On top of having different teams supporting each OS, because a company developing a project this large isn't going to have a small team where the developers kno

On top of having different teams supporting each OS, because a company developing a project this large isn't going to have a small team where the developers know each OS (not quite full-stack in the web development stack, but just comparable to developers that know every OS they need to build for). There then needs to be communication between different development teams, as well as offering feature-parity across OS's, because your clients need to be able to communicate with each other across OS's since your product is a chat application for business. It's not normally acceptable to have features that are different between OS's when your product is meant for team chat.