It is indeed cross-platform. Of course, your client might be running an ancient PC or Mac. Or they may not be able to figure out (how many times I have had to deal with this one) they need to DOWNLOAD the file, not try to play it from their mail client.
Or they tell me they can't play the file, when what they REALLY mean is they can't FIND the file. They don't have any idea where it went when they did download it.
Or they tell me the file won't play, only for me to find out they are trying to play some file other than the one I sent them.
Dell Precision T7600 (x2)
Win 7 64-bit
Adobe CC 2015.02 (as of 6/2016)
256GB SSD system drive
4 internal media drives RAID 5
Typically cutting short form from HD MP4 and P2 MXF.