Take for example this first-order rule:
First-order rules are not too complex. There is usually one variable that is different between the two sets. Here, it is the curvature of the sides of the shapes. These rules can be complex, but only on rare occasions. The difficulty in rule finding is when another layer of difference is combined. Hence second-order rules.
The majority of Abstract Reasoning rules between Set A and Set B that you will encounter in the real UKCAT test are of two-orders of rules. This is where more than one variable difference is used to build rules. For example,
As you can see from the example above, there is an additional rule which is somewhat unrelated to the first part of the rule. Second-order rules such as these are used throughout the test. This is why Abstract Reasoning can be difficult.