Good grief, you posted something worth reading
Except I think you got it slightly wrong...
Deep sea bed burial of vitrified waste is good. Very deep sediments, high stability, low permeability, inaccessible.
Subduction zones however are inherently unstable.
Cons to deep sea bed burial:
Very costly, difficult to drill in deep sea area.
And oh yeah, there is an interntional treaty that prohibits dumping of waste at sea. Even the US has signed it.
(Not that that is a problem these days)
Cheers,
Tony