Power Line Communication (PLC) devices are increasingly used and available. However, research carried out at the Medium Access Control (MAC) layer is limited. This article addresses Quality of Service (QoS) mechanisms defined in the widely implemented Homeplug and IEEE 1901 standards. By means of a testbed constructed from off-the-shelf components we identify a number of issues with a potentially significant impact on user satisfaction: i) a pronounced starvation and variability of lower-priority traffic when different access categories are combined and ii) an oscillatory behaviour in higher-priority and high-traffic configurations. We also determine the underlying causes of such findings and propose possible solutions. Our contributions are of relevance to both the research community and manufacturers, as we identify crucial aspects to be revisited in order to guarantee successful advancement and further adoption of the technology.