The key to your answer lies in the fact you have a single relay controlling several others. If one of the several fails, chances are it's not going to cause a major issue as everything else will still stop, but if the single one fails it's more likely to cause major issues.

However a correctly designed system, the relays should be suitably sized that contact welding is not an issue, and suitably fused. Safety relays aren't entirely fail safe, they've just got extra contacts so you can check to see if they're working correctly.

It's all a case of risk management and cost.
Ideally safety systems should be tested reguarly, to ensure they are functioning as designed, and any problems found before they cause problems in the case of an emergency.
However, if you have something that needs to run 24/7, or where regular testing could be a problem, then you want to minimise the amount of testing that needs done, and look at using more failsafe methods.

In the context of a homebrew machine running Mach/Linux, you want a couple things to happen during an e-stop. First you want to cut power, and second you want to the tell the computer things have stopped. Should the power fail to cut, then the computer should still know things have gone wrong and stop generating pulses. That way should the main power not die, the computer should still stop commanding any movement. Plus you always have the completely failsafe method of unplugging it.

Industrial machinery does get a bit more involved, but there are no hard and fast rules on implementation, just that things should stop in a controlled and quick manner. There are stipulations on time taken to stop things like exposed cutters, but there is no requirement for power to be cut, just for movement to stop. However generally anything that can be stopped faster under power, will use a time delay system, which when activated, will issue a stop command, then after a short delay, cut power.

One thing to remember, nothing electrical is ever truly failsafe, and is why you should still be able to manually kill all power should things go totally wrong.