Adding carrier sense capabilities to nodes in lowpower wide area networks is considered as a good strategy to cope with scalability problems. However, in settings with large cells, the clear channel assessment function will give imperfect results, producing scenarios rich in hidden nodes. In this paper, we assess the impact of these hidden nodes, showing that the average gains with respect to an Aloha strategy are significantly decreasing as the number of hidden nodes increases. Moreover, in terms of energy, our results indicate that carrier sensing consumes 10 to 100 times more than a simple Aloha channel access. We also look at the individual node behavior, demonstrating an important heterogeneity among nodes, with performance generally correlated to the quality of the carrier sense mechanism.