I have been using an H-style cat harness with no regrets on my young energetic neutered male (age nine months when I got him) for six months. The H-style harness is two loops, one around the neck and one around the waist--I would say under the armpits. Width: 1 centimetre or 3/8th inch. I read that the loop under the armpits, which I will call the body loop, should be so snug that you can barely get one finger underneath. The loop around the neck seems not to matter if it is a bit loose. Both loops can be adjusted for size.
The two loops are connected on the back with a strap, the cross-bar of the H. On my harness, that strap is not adjustable. Thus, as my cat grows longer, I have had to adjust the neck loop, making it larger and larger. It is now larger enough that I can slip it over his head before I buckle the body loop. When the body loop (that goes under his arm-pits) is buckled, his head cannot slip out because the back-strap is much too short. The back-strap is reinforced and holds the two loops closely together. It even pulls the body loop toward the head, making it impossible for him to slip out.
At least, I don't think he can. I've been told (by a person with no experience) that he could but so far he hasn't. I've seen him hanging from a tree because he tried jumping and was too far from the ground. Not something I want to ever see again but surely he would have slipped out then if it were possible. And there have been other times that I am sure he would have slipped out if he could. Or maybe it's because he was trained from so young an age that he can't.
However, I think having such a narrow band harness passed snugly under his armpits and over his back makes it seriously difficult to slip out. Besides, the neck loop holds it in place and the leash attaches to a ring on top of the body loop. I think it's back a bit farther on his shoulders than the one in your photo. That, too, should make it more difficult to slip out because when you pull on the leash his weight rests on the body loop behind his elbows. He can't get out so long as his elbows are outside the body loop.
My cat's lifestyle is very similar to yours and I feel it was definitely worth what I paid for it. I recommend it. It appears comfortable, too.
There's one thing that is not so pleasing esthetically. Now that he's grown so big, and the neck loop is so large, the back-strap often slides to the side. This causes no problem at all that I have found but it looks odd. The other thing that has happened once or twice was the snap coming loose from the ring. I don't know how that happened, maybe I twisted it the wrong way so now I try really hard to be careful about that, especially when he climbs trees. It's a swivel snap and I've had that happen a few times on my dog's harness, too. I suppose it's one of the risks of taking pets outside on a harness.
NOTE: The H-style harness should not be confused with harnesses that are very similar but have two straps connecting the neck and body loops. On some of these harnesses, the connecting strap(s) is/are adjustable. I cannot speak for these other designs. There must be important differences when it comes to restraining cats.
PS See Allison C's comment below regarding other styles with two loops. She says they are all "fairly secure."
