Google "reliability" seeking the answer on any format will bring you conflicting answers. Simple answer is there's probably no total guarantee of reliability on any of them.
I prefer External drives because of size, up to now no problem, then again I do have three which contain backups of backups.
In addition photos on CD or DVD are kept from before externals. Recently I tested a couple of cd's containing files from way back and they had degraded to being useless.