Large Language Models have shown prominent capabilities in generating functional code from natural language descriptions. However, a standardized way to evaluate these capabilities in an objective and unbiased manner is still to be found. In this paper we review the current evaluation methods available to this end, and run a new evaluation of the performance of one state-of-the-art model (GPT4-o-mini) in solving curated coding challenges in 8 programming languages, obtained from Codewars, a software development community. Our analysis shows that the chance of success of the model has a positive correlation with the task difficulty, the popularity of the programming language being used and the time elapsed since the publication of the challenge. A further approximate explanatory analysis in terms of high-level features hints that while 46.6% of the model performance could be attributed to task difficulty, a 37.4% seems to be related to leakage of the challenge solutions into the model training set, while the remaining 16% depends on the programming language. These results suggest that current evaluation methodologies might be overestimating the actual skill of Large Language Models for generating functional code.