The reason often given as to why ball valves are not "acceptable" on thru-hulls
is that they lack the support that seacocks have. That support MIGHT be
needed to close them in an emergency and could possible break off (with
disasterous results unless you had some tapered wood plugs handy). Yet many
boats come equipped with properly functioning ball valves. I think the key
here is make sure your ball valves continue to properly function by actuating
them once a month or so and lubricating the top and bottom of the ball at
least annually. If they are working properly, they are probably safe enough.
Certainly safer than a seacock that has not been maintained....