It's inefficient after a decade or more of AltTabbing in Windows becomes a muscle memory that is thought-flow breaking when having to un-train that automatic keyboard reflex.
It switches between applications, not windows.
Nothing's wrong with it, unless you prefer alt-tab to switch windows. I personally prefer to choose my own adventure.
Imagine you’re on a German keyboard and want to switch between Firefox windows. In that case I need to do cmd + shift + ` which need two hands since ` is in the upper right corner. That’s the only use for me: switch between windows of the same app easier.