The most frustrating thing about FaceTime is it sometimes appears to significantly duck audio in order to avoid echoes. I can't predict on which devices it will happen, but it often does when I call my parents and it absolutely destroys the conversation. If they're telling me something and I make the slightest "uhuh" acknowledgment sound, their mic input gets effectively muted for a second or so and I miss what they say.
I’ve found that if the recipient is wearing headphones/earphones, you can freely interrupt them without getting ducked (and vice-versa). Doesn’t help much when you’re calling multiple people / have to be on speaker, but makes it predictable at least.
Well sure, that's the easy case for echo cancellation (remove the source of echoes entirely).
But we've had solutions to this for decades and it doesn't involve ducking the recipients audio. Apple and their billions of dollars should have had this completely solved by now.