Licence

Many wiki engines are Free and Open Source (FOSS), but the exact terms of the software license may vary. Some projects come with their own customised license terms, but most use standard licences such as GNU General Public Licence (GPL) or Lesser General Public Licence (LGPL)

There are a variety of possible restrictions which the software licensing may impose on you. For example, even if something is Free and Open Source (FOSS) you are not normally allowed to modify the code and redistribute it as your own work, without crediting the original authors!

Just because code is open for the community to develop, it doesn't necessarily mean anyone is doing so! You may want to check if there is an active development community, whether they are accepting changes to the code. A very old Last release date might indicate that the project has gone stagnant.

Obviously this refers to the software distribution, not the textual contents of their wiki homepage, or of your wiki installation (you need to think about content licensing after installing a wiki)