It seems to me that there are a couple of separate considerations here, with regard to how safe an action is. Gas venting is only one of them. Others include the extent to which the action actually contains and supports the case head, with some leaving more exposed/unsupported than others, thereby increasing the likelihood that gas may actually need to be vented. The Winchester M70 and the Springfield are examples of actions which allow more of the casehead to be unsupported than, say, a Remington 700. All well and good as long as the case does its job, but a potential failure path if the case is defective or overloaded. An action which more or less envelops the case head may reduce the likelihood of the case actually letting go in the first place. As well, it may be better able (and failures I've seen support this) to contain the product of a case-head failure rather than directing it out of the action.
Having said that, I think the risk of a case head failing is rather less now than it was when Mauser was designing the 1898 model.
As well, there's how well-supported the primer is, with some actions having a larger amount of clearance around the firing pin than others, and in some cases a more or less direct path to the shooter if the primer fails.
Shear strength of locking lugs is also a factor, but perhaps more important than strength is the work of fracture of both lugs and receiver. A safe receiver, even when grossly overloaded, should bulge and distort plastically, thereby absorbing energy rather than flying to pieces. A brittle failure is much more hazardous because even though the strength may be there the ultimate failure mode does not absorb appreciable energy but instead releases it into flying pieces of rifle. As well, if the rifle really is to let go, the design should, to the greatest extent possible, prevent the bolt and other parts going back into the shooter's head. You see such thinking in the design of actions like the 1898 Mauser, with a lug in reserve intended not to engage when the action's locked, but to prevent the bolt going back into the shooter if the locking lugs fail.
