Despite being very generic, TableGen has some deficiencies that have been pointed out numerous times. The common theme is that, while TableGen allows you to build Domain-Specific-Languages, the final languages that you create lack the power of other DSLs, which in turn increase considerably the size and complexity of TableGen files.
At the same time, TableGen allows you to create virtually any meaning of the basic concepts via custom-made back-ends, which can pervert the original design and make it very hard for newcomers to understand it.
There are some in favour of extending the semantics even more, but making sure back-ends adhere to strict rules. Others suggesting we should move to more powerful DSLs designed with specific purposes, or even re-using existing DSLs.
TODO: Add here frequently asked questions about why TableGen doesn’t do what you want, how it might, and how we could extend/restrict it to be more use friendly.