I guess it depends on how you were trained to ride. I come from the western riding world and you'll be hard pressed to find any other curb as mild as a kimberwicke! It is practically as mild a curb bit as you can find!
I own a couple of them and find them pretty mild. Of course, I ride on a loose rein too, and never give constant pressure on the reins. In other words, I touch and release and ride on a loose rein, and hardly ride with contact as many English style riders do (unless trotting or cantering, then I usually have a little contact).
I sometimes ride in bits with A LOT more leverage than a kimberwicke, similar to the one the original poster posted a picture of. It's a little long on the leverage, but not horrible by any means.
I don't find that riding in a leverage bit means you can never go back to a snaffle. As a matter of fact, if you use a leverage bit for a while and then go back to a snaffle, the horse is often lighter in the mouth. I flip flop around with my bit collection on my horses all the time. They are very light in the mouth and I haven't ruined one yet. I would venture to say that my horses are as light or lighter after I have been riding them for years then when I first bought them.
So I don't know why tom thumbs and kimberwickes have this reputation of being evil.
If a person rides with sensitive hands (and you should with ANY bit) then what is the big deal? Let the horse be the judge.
I consider a horse to do well in a bit if: 1. I have the control I need
and 2. The horse seems comfortable in the bit and gives to it without seeming uncomfortable or tossing it's head or mouthing it excessively
and 3. The horse and I can accomplish numbers 1 & 2 even if the horse is excitable for some reason, (such as riding down a dirt road and a pasture full of horses comes galloping up- then you quickly find out how your horse responds to your bit). In other words, I have a comfortable amount of control when I really need it, AND the horse seems comfortable in it.
If you "over bit" a horse they will let you know. You won't be able to make contact with the bit without them showing agitation.
I try out a lot of different bits when I trail ride, from snaffles to curbs to hackamores, and I have not found kimberwickes OR tom thumbs to be harsh bits.
By the way, there are two (or more) types of kimberwickes. You have the type with the solid mouthpiece and a port, and the type with the broken mouthpiece (like a snaffle). I don't consider either one evil!
Try out different bits and let the horse be the judge. See what gives you control AND is still comfortable for the horse.
I admit one of my pet peeves is blanket statements such as "bit XYZ is evil!" :roll:
One of the greatest joys of riding is having a powerful, energetic horse under you and being able to regulate that power with just the touch of your fingers! So I try to find the bit that gives me that feeling of collected power with my horse. I see a lot of people riding in snaffles because they are "kinder," but then they are constantly riding the brakes and pulling on the horse. I would rather use a little stronger bit and have the horse respect it, and have fingertip control than have to pull on the horse with a plain snaffle.