Any engineer will tell you "the harder the steel the more brittle it becomes". Soft steel will 'bend' and ultra hard steel will 'snap'. When making secateur blades a compromise must be reached: too soft and the blade won't keep its edge, too hard and it may become too brittle and snap under pressure.Every blade manufacturer strives for a different point of balance in the hard/soft continuum. As a general rule the cheaper the secateur the softer the steel used in the blade - and the vice-versa is also true. Soft blades are more forgiving (of mistreatment) and, in general, the user of a cheap pair of secateurs is less demanding re the sharpness of the blade edge. Premium secateurs always sport a hard blade.
As an example, Felco blades are made from high carbon steel to achieve their hardness and, over the years of manufacture, Felco have managed find the right balance. The simple fact is, a Felco blade will stay sharper, longer - but it will cost more to buy in the first place. Even if the edge is dulled a few passes with a Swiss Istor sharpener and it can restored in a less than a minute. Having said that, a Felco blade does not like to be twisted in the cut (due to its degree of hardness).
A quick way to tell if you are putting too much lateral force on a blade is the check the blade edge after a few cuts. If the fine 'edge of sharpness' appears slightly (and I say SLIGHTLY) burred or uneven you can almost guarantee you are giving the secateurs a minute twist with your hand mid-way through the cut. The right pruning technique will ensure this problem does not occur.
Treetools advocate the use of a Swiss Istor sharpener on your secateurs (hard or soft bladed) at every break in your pruning work. That way you will keep the edge in optimum condition.