In this paper we investigate the impact of patient's profile on the usability of filling clinical questionnaires on smart devices. Our study was conducted in an oncological day hospital, where the EORTC QLQ-C30 quality of life questionnaire is administered to outpatients using computerized devices (smartphone, tablet, and laptop) and a paper form. We show that all the devices are evaluated as usable by the patients, under the dimension of easiness of use, and provided information. Moreover, we show that the patient's cognitive functioning (CF) impacts negatively on the evaluated usability of laptop-based surveys, suggesting that CF must be taken into account in the GUI design. Finally, we illustrate that the patient age and her technological skills also have a negative impact on the evaluated usability.