What makes the information contained in the artwork of Late Middle Ages reliable and/or not reliable?
In other questions that you have asked, you have been mainly concerned with fashion. Therefore, I will answer this question with respect to fashion.
The main reason why the information contained in the artwork of this time is reliable is because it is contemporary. When people paint pictures that depict their own time, they are much more likely to get the clothing styles right than they would be if they painted pictures that were set 50 or 100 years previous. To the extent that the artwork is set in contemporary times, it is a reliable source for what people wore.
However, there are at least two main reasons why the artwork may not be completely reliable. First, it was not painted mainly as a way to document fashion. Instead, it was painted in part for aesthetic reasons. This means that paintings might show people wearing things that the painter likes rather than things they actually wore. Second, the artwork was typically painted for rich patrons of the arts. This meant that it portrayed mainly upper class people. Therefore, it might not be reliable as a depiction of what regular people wore on a daily basis.