I use FolderMount with next to no issues.
However, it would not automatically move the files when linking folders so I manually moved the files from internal to external THEN linked the folders. After that no problem with those folders except;
DCIM folder link caused my Camera to become unavailable and/or all photos taken were corrupted. Once link was removed all worked normally. Now using Dropbox w/ 50gb and it automatically saves all my photos, so I just removed them fron internal memory after that, freeing up a few gbs