My point was intended to be that there's the same challenges and more - but it's not something I've thought about in depth (and certainly not had to work with), it maybe wasn't a very good characterisation because it's not the same on the other side either, no large file to serve because at the start of the call it doesn't exist yet for example, so perhaps I take it back.
It is much much harder because you can't do the "cache everything on the edge" solution. If storage was infinitely cheap and small, Netflix could run their entire business by sending a USB stick with every single movie/tv show they have on it encrypted to you, and everything would play locally. This is basically what they do with their edge servers/CDNs.
You can't do that with video calls, because the video/audio didn't exist 1 millisecond ago.
In any case, it's hard to say what the 'greatest' engineering challenge is. You can make almost any kind of engineering really challenging, if you (or the market..) sets yourself a very low cost-ceiling.
I'm not sure what you mean? Real time communication, both video and audio-only, have much lower latency requirements. You can't just buffer ahead when you have some spare bandwidth, like Netflix or YouTube can.