Yeah, I can only guess at what OFP does behind the scenes and whats involved with things like the Alive command.
Another side effect of using the @ command is the scripts run at the current frame rate. I have not had chance to test if this is the same when using ~, although I do know if you tried say ~0.0004 it would never run any faster than the @ command, which on my PC executes every ~0.03 (on the desert island).
I will stick my neck out
and say as a rule of thumb only use it on variables, and then nothing more complex than:
@(Call {Boolean1 Or Boolean2})
But then, it wont be the first or last time I get it wrong
:-X