The biggest problem is the animation in the screen.
I get a blue popup in the bottom corner saying a meeting has started, I click join meeting and a popup with settings appears. More often than not the microphone button shows as gray, I hit the join meeting button and it takes a minute for me to notice that my mic is not muted.... The mute mic button/switch has an animation on it that starts as 'off' then animates to the blue on position. So if you click through the setting popup quickly to join the meeting you assume mic is muted cause you didn't wait long enough for the animation to finish.
Seems like a lot of people have this issue. Every time a meeting starts all we hear is background noise from people in the office sitting around those on the call.
If everyone was muted by default we won't spend 5 minutes asking everyone to mute and trying to figure out who is sitting next to a xerox printing 100 books.