Within this work, we aim to identify increasingly challenging reactions, i.e., reactions for which the activation energy exhibits the highest variability across several functionals. Two common quantities can in principle guide us to this end, the range and the standard deviation (σ) of the computed activation energies. We considered the range to be less useful here, as it does not convey any information regarding how dispersed the values truly are, i.e., a high range can be obtained because a single functional is an outlier for a given reaction data point, while this datapoint may still result in a narrow distribution of (activation) energy values overall. σ on the other hand is a direct measure of how dispersed the values are around the mean. For this reason, we selected it as the target quantity throughout this study.