I have used the functionality to add album art to flac tags and that works very well, but it is extremely slow. It seems as mp3tag rewrites the whole FLAC file just to add the album art tag. Is this really necessary? It takes several minutes to tag a FLAC album with album art. With mp3 it just takes a few seconds...
This is not possible because ID3v2 tags are always written at the beginning of the file (except for ID3v2.4 which can also be written at the end, but there is currently almost no software or hardware which supports such tags, except for Frontah).