Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. It was born in wherever your camera, screen recorder or download came from, and its real weak spot is nothing inherently; every container brings its own quirks.
M4A is AAC in Apple's favorite outfit: the audio-only MP4 your iPhone speaks natively. An MP4 container holding only audio: usually AAC, occasionally Apple's lossless ALAC. That makes it the right destination when what you need is sounding great anywhere in the Apple ecosystem.