They are good as long as the chain cannot touch the ground. A D lock is easier to angle grind or break with a bottle jack, a chain is easier to break with a hammer and anvil or bolt croppers. Angle grinders are rarely used but small bottle jacks are quick and silent if enough room in the D has been left.I do use a motorcycle chain lock on my bike, despite it's weight. I think these can be even better than a D lock since to hold a link still for angle grinding would be extremely difficult, while a D lock is much easier to angle grind.
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The metallurgy used on the locks is very important and changes the strength massively, you can't see this and sometimes when manufacturers change their suppliers quality can vary for the same model of chain (or D lock).