I find the kimberwicke a very useful bit for my haflinger. When we are in a controlled environment I can ride him bridleless because he will work completely off of seat and voice aids. He does fine in a snaffle as well, but he is not a fan of a single jointed mouth piece much at all and gets moody. French link is a little better but not much. I have found he just does not like the jointed type mouth pieces. Maybe this is the OPs horse's problem? If that is the case I would look for a low or medium port instead of the jointed mouth piece. Maybe try borrowing one before ordering it to make sure that it does work. In the years trying to find what make's Toby happy I have done lots of tack exchanges and bit borrowing before finally finding something we were both happy with. I'm one who went from a "have to work all horses either bitless or in snaffles to be the most humane" to "use what works best for that horse/rider combo."
Now he does excellent in his kimberwicke. We have a low port and mouth on it and he is very happy in it. The small amount of curb on it is minimal compared to a short-shanked colt bit or pelham but it is just enough for when he gets a bit strong when riding in a group or tries his occasional dive for the grass out on the trail. With good hands and the right horse - this bit can be a good choice.
I know some prefer a pelham instead of a kimberwicke, but the thing is - how many people are really proficient in using a double bridle? It takes a while to get to that point and most of your low-level riders or basic pleasure trail riders just are not to that level. A kimberwicke is an alternative that gives you a bit more than a snaffle, but not as much as an actual curb. I would rather see someone using a kimberwicke than totally messing up the cues with a pelham because they can't keep the double reins straight.