Mp3tag is a Windows program only, so you cannot really talk about cross platforms.
As for what dano said, I find it weird that Mp3tag synchronises the tags only when adding or removing fields. If you change an existing field, the tag doesn't get removed. I think the same thing should happen regardless of the operation. Example:
Take a file without tags. Configure Mp3tag to read and write ID3v2 and APEv2 tags. Add a title, an artist and a cover. If you view the file now, you won't see the cover because it's not in the APEv2 tag. Now disable reading APEv2 tags and view the tags again - the cover is there. Now switch writing of APEv2 tags back to on and modify the title. The new title gets written to ID3v2 and APEv2 tag, but the picture inside the ID3v2 tag is still there. If you, instead of modifying an existing field, add a new one like album and then save, the cover also gets removed from the ID3v2 tag.