The main benefit of general over niche is traffic. General directories will usually get more traffic.
The beauty of niche directories is they usually gain targeted traffic. i.e. the people on that directory are either interested in submitting a site that fits the niche, or looking for sites that are realted to the niche-topic.
If you have a niche that is too specific however, you run the risk of not having enough traffic to remain viable. I think for longevity you need to aim for popular or broader niches.
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