The problem isn't E2E encrypted group video calls. FaceTime supports that. The issue is interoperability with E2E encryption.
If Apple says sure, implement this FaceTime spec. Facebook does the same thing, go ahead and implement Messenger video chat.
Now you have the Android NewVideoChat app which supports its own protocol, Facebook's and Apple's. A user with NewVideoChat tries to invite a NewVideoChat user, an Apple user and a Facebook user to a video chat.
Except Facebook Messenger's app doesn't support Apple's Facetime app doesn't support Facebook Messenger, so you run into some issues. Something needs to dupe the stream out to all three services which use radically different payloads and encryption methods - and they have to do it without breaking end-to-end encryption. Do it at the client-side and the Android app users will need to dupe their own streams three times and at least one user will need to relay the other two other streams, with all the bandwidth and latency issues that entails. Do it on the server side and you somehow need to translate between protocols (and possibly codecs!) without decrypting them.
And if your video group chat supports private messaging between a subset of participants, you can end up in a situation where a Facebook user wants to send something to a Facetime user without the NewVideoChat user seeing it.. which is a bit of a problem.
The problem isn't E2E encrypted group video calls. FaceTime supports that. The issue is interoperability with E2E encryption.
If Apple says sure, implement this FaceTime spec. Facebook does the same thing, go ahead and implement Messenger video chat.
Now you have the Android NewVideoChat app which supports its own protocol, Facebook's and Apple's. A user with NewVideoChat tries to invite a NewVideoChat user, an Apple user and a Facebook user to a video chat.
Except Facebook Messenger's app doesn't support Apple's Facetime app doesn't support Facebook Messenger, so you run into some issues. Something needs to dupe the stream out to all three services which use radically different payloads and encryption methods - and they have to do it without breaking end-to-end encryption. Do it at the client-side and the Android app users will need to dupe their own streams three times and at least one user will need to relay the other two other streams, with all the bandwidth and latency issues that entails. Do it on the server side and you somehow need to translate between protocols (and possibly codecs!) without decrypting them.
And if your video group chat supports private messaging between a subset of participants, you can end up in a situation where a Facebook user wants to send something to a Facetime user without the NewVideoChat user seeing it.. which is a bit of a problem.