The probably chief cause of keel bolt failure is: they are not
removable for inspection.
Stainless steels are 'very' subject to 'crevice corrosion' and fatigue
failure, mostly a combination of the two modes of failure. Worse, the
maximum stres on a keel boat is 'cantilever stress' ... very hard to
predict since the loads and actions are 'variable to unforseen'.
Aiarcraft have the almost same problem with cantilever stress ... but
those wing root bolts are REMOVEABLE and thus are able to be
periodically inspected.
So, keel bolt failure is a functional design failure due to the bolts
not being able to be removed and periodically inspected.

Encapsulated (iron) keels are not a panacea, as if the cavity is
penetrated and water enters, the rust (ferric) that forms is less
dense than the original metal (iron) and the encapsulated keel will
'push' itself apart ... then the balllast will simply fall out if the
rust formation is that great.

So, from the above it seems that a encapsulated keel with solid lead
internal ballast would probably be the 'best'.