I make harnesses and use the heavy duty industrial strength velcro. I've picked up my 7.5 lb girl by the leash attached to the dring and it has never even started to come undone.
I think the main thing to look for when getting velcro harnesses, is that there is a lot of velcro used, and that it's good quality velcro, and that the dring is securely sewn.
I even have a 12.5 girl who wears my velcro harnesses and she's a puller. I've never had any issues with it, and I heavily tested it with a back up just in case! I am so paranoid