User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-172.56.23.54-20201214223046/@comment-1038387-20201215223059

It works on the surface. But every fix requires so much working around custom coding and outdated core that it is incredibly inefficient. From a business point of view, inefficient equals broken. Many backported features are innate in the new software, so it runs more efficiently.

But if you want one example: Forums and threads were hacked MediaWiki, and it showed in the maintenance categories. If someone added a category to a forum or wall reply, it would categorize the reply's underlying name. Not the actually visible name, so you could not edit that link directly, you had to decipher the time stamp from the url and then look for an edit on that board or wall from that day and time. The same goes for broken and wanted images. And because of how the API works, this was not something a bot could clean up for you. And oh yeah, this categorized reply? It would not actually show up in Classic Categories or Dynamic Categories, only in Category Exhibition. In the other two, it would just show up as an empty entry under T for Thread.

Hacked MediaWiki is full of problems, and the WikiMedia Foundation tells you not to do it.