With regard to safety: Fire might cut or melt the PoE wiring isolating people. Why would you want to use PoE rather than dedicated armored cabling and controls?
Definitely a concern and not to be overlooked in the design and layout of the PoE solution. Options include separating PoE paths, using redundant PoE switches, and even conduit to protect certain feeds of the Category cabling. Just like traditional emergency lighting systems flaws do exist, the challenge is weighing the good and the bad to determine which solution best meets the need of the current project.