Oh Boy, wow lots of very good points, probably too many to cover, so i will just give my POV.
Sheilds are a risk no matter what they are made of, too rigid a material like wood and you have the danger of being smacked by one that has just been swung by someone not able to control it.
too flexible material and you get no protection from the sheild which defeats the purpose.
I have used Corflute sheilds a lot in Larps, and will never use them again since the last one i used at a Mordavia game tore MY knuckles to peices after one of the enarms (Straps) was torn from a blow.
I was using a wodden sheild at STWVH and in the end I ditched it, it was unfamiliar to me and heavy which meant i was not sure how it would behave in combat, the risk was too great so I ditched it and switched to case.
safety with a sheild comes down to the skill of the user, just like any weapon, if you use it in an unsafe manner then it will be unsafe, if you are careful and use the sheild safely it will be safe. Now of course you cannot count on the actions of others, but as far as I am concerned if some muppet decides to perform DIY dentistry on the edge of my sheild then it is thier problem.
The best sheild i have ever used is the one i have in my room, It belonged to Steve, it is an East German Riot Police sheild, flexible ABS plastic, with thick soft rubber edging, light, strong, easy to use, and safe.
Derek used a wooden sheild the whole weekend and did anyone have any problems with the way he used it, did he hurt anyone with it?
take it at a case by case, game by game basis by all means but dont just automatically default back to Corflute, in my experience it aint that safe.