[FFmpeg-user] Re-packing video in new container file really working for all container types (AVI, MP4 MKV, WMV) and codecs?

[Ben]

As I read I can lossless re-package an existing video from a source container file into a different target container file by issuing e.g. the following command:

FFmpeg.exe -i sample.mp4 -acodec copy -vcodec copy sample.avi

or

FFmpeg.exe -i sample.avi -acodec copy -vcodec copy sample.mkv

So the target container type is simply identified by the target file extension.
No additional parameter is required.
Correct?

"Lossless" means without re-encoding the video and audio content.
Meta data values are obviously lost when "downgrading" e.g. from MP4 to AVI.

Can someone please confirm that the target file (AVI, MP4, MKV, WMV) will be full compliant with container definition and not just a dirty workaround from ffmpeg.

Is this lossless re-packing possible for all types of video codecs (e.g. H.264, H.265, VP9 and Xvid) and audio codecs (MP3, AAC) or are there any restrictions
which require a re-encoding anyway?

Thank you
Ben