It's a shame you chose FF as an example since that was the map I was talking about.
In the vlog they explained it really well. Due to the cap points being so close to each other mesmers can set a portal on GY and then run back in their base or to one of the other cap points.
This means that you have to run with your entire group at all times otherwise you'll be fighting 5vs(<5) meaning that you will always lose.
This also means that it's easy to deal with gate rushers since you're basically camping your defensive point anyway (or attacking their base)