Many of my clients have struggled for years to kick well with a kickboard, with their coaches just wanting them to kick harder. I believe that lots of kickboard sets makes it very hard to learn the essentials of efficient kicking.
Your head is usually high, putting pressure on your lower back, sinking your legs, and forcing most people (triathletes and runners, especially) to kick from the knee, not the hips.
Kicking so that you move forward with any kind of decent speed is very hard work because of the drag your legs create. This makes it harder for you to kick with good technique for more than 25m without getting too tired. To improve your swimming stability, you should be able to kick effectively all the time, not just for 25m.
Kickboard tip: To minimize the poor posture of kickboard kicking, extend your arms in front of you so that your head can rest face down in the water. Also, use the smallest kickboard available so that your upper body isn't floating so high.
Face it, you're a land creature and you're built for bipedal locomtion...walking, running and our clever ape-descend genius-inspired activities like cycling, step-aerobics, and.... burpies.