I agree 100%, Felix!!
It's always best to book with an agent because they're able to explain all the nuances of a particular policy and answer all the questions so the client understands the policy and makes sure it covers them for their specific needs. As with anything else in life, all insurance companies and all policies are not created equal and it pays to have an agent who will help you should problems arise.
Booking through these large online sites does not provide the personal touch when needed because the overwhelming people you talk to work in a call center and are not certified travel agents. They just sell, not provide service.
As you alluded to, the majority of complaints on that website did have a common theme, but the problems mainly dealt with the fact the did not understand what they were buying, didn't ask any questions, and assumed it would cover everything in every situation. Not every policy is for every person and it's always good to have someone on your side.