The last New Scientist had an article on this. The primary unknown/concern was structurally destabilizing the seabed by extracting the methane. They were talkin

The last New Scientist had an article on this. The primary unknown/concern was structurally destabilizing the seabed by extracting the methane. They were talking about replacing it with CO2 to keep the seabed stable, but said that was really only an idea and just extracting it was safe, you know, until the massive tsunami they cause kills 100,000 people <morbid joke/>