I own eight plunger style inlines. Two of them use 209 ignition systems. I see nothing dangerous about them. Now my Whites use #11 although I could put the 209 breech plug in them, but choose not to. The Black Diamond XR could use any of the three ignitions and again, I use 209 primers. I have shot hundreds of rounds through the rifle and never saw any problem other then when I used BlackHorn 209 powder. But note, the powder was not made for that rifle. My CVA Stag Horn Magnum uses 209 and again, I have not seen any problems with it either.
I am curious Grouse as to what you find dangerous with a plunger style inline using 209 primers.