In reply to GridNorth:
Hi, I guess it depends on what you mean by safe, and how much experience of the act you have at the time.
1. when you leaver the thing with a krab it can release fast. How fast depends on the friction (e.g. whether the rope is taking a zig-zag through runners etc.) So the question is has the person tested this in a series of settings.
2. yes, if you let go of the krab it will lock. But will you let go? Letting go is not instinctive, gripping is. This has been found when belaying with a grigri - people grip the climber's rope rather than letting go. They get burnt, climber decks.
3. climbing in a three. This needs people to read the instructions that come with the device. As you lower the second, the third will be unbelayed, so had better be backed up.
Conclusion. If you are lowering, use a back if new to it, possibly not if experienced.
So, what is meant by experienced. One way to think about this is for each activity state what N, F, and C are. Where N is the number of times you think someone should have done the thing to be not get it wrong more than F percent of the time and C is the consequence of getting if wrong. If C high, probably best if F small.