It comes down to this: Show us the bodies. It applies to everything that could be perceived dangerous. Second Hand Smoke, Steroids, rear-fanged snakes, etc. Mangroves have been kept in captivity by lots of people. If they were really dangerous, there would likely be evidence of death/injury.

Perhaps the venom is dangerous but the delivery poor. Perhaps it's simply that they aren't typically a "handling" type snake so dangerous bites don't happen much due to little exposure. If you're thinking of getting one, gloves, goggles and a long sleeve sweatshirt is probably good enough to protect for cage maintenance and most other interactions.

Doug T
Doug Taylor Reptiles