I've had zero problems getting the KB unzipped. You do have to practice the draw until you're not sweeping your arm but it's not difficult. I'm guessing (and correct me if I'm wrong, Mainsail) that you've never actually tried a HPG Kit Bag and are just basing this on the video and/or thought experiments. It's really fast and easy to draw from. I agree that it's hard to beat a naked holster on a padded pack belt but that's not an option for everyone isn't even legal in many places. IIRC in Texas you can CCW but not OC a handgun.
No, I've never owned one. I used to carry a Ruger Alaskan in a chest holster but found the gun grossly heavy and the gun & holster terribly uncomfortable. No matter how hard I tried, and I really tried because I had so much $$$ tied up in it, I just couldn't get used to anything on the front of the pack. Sold the gun and gave the holster to someone here in the forum.
Other hikers with whom I hiked tried carrying their camera gear in chest packs and found them too hot and uncomfortable. It doesn't have to even be a hot day and I have sweat running down my back where the pack lies, now add another pack to the front? For a casual stroll maybe, but for day-long hikes with steep terrain, you'll end up soaked and dehydrated. Front packs are nothing new, but you don't see them on the trails much- there's a reason for that.
Once I went to the Bianchi M12 on the pack belt I found it far more comfortable, safer, and easier to access. I actually have two of them, one has the flap and the other has the thumb-snap in place of the flap.
I have nothing against the HPG pack, but for where I live there are better options. I think the HPG is a LOT better than what some hikers do- put their gun inside their pack. We had an incident with a very drunk Native American on horseback who held a small hiking group at gunpoint, all the while screaming at them and making threats. One of the hikers had a small handgun inside his pack but no way to get to it under those conditions.
Like anywhere else, situational awareness is the best thing you can have.