Unionfs vs Aufs vs Overlayfs vs mhddfs, which one do I use

I have randomly been reading about union file system which enables a user to mount multiple filesystems on top of one another simultaneously. However, am finding trouble deciding on which one to use(Unionfs vs Aufs vs Overlayfs vs mhddfs) and why as I have not found concrete information on the subject anywhere. I know for instance that overlayFS has been adopted in the mainstream Linux kernel which means it might get wider adoption. Would appreciate if someone would give me some perspective. Also I can't find any conceiving use-case for Union file system over something like LVM (as recommended by users in separate question ) or RAID setup except in the fact that LVM requires formatting all the drives which might not be desirable if you already have valuable data on the drives.