Since I'm using TM1 as an old version of Planning Analytics, it wouldn't be fair to specify what needs improvement because it's possible these issues have already been addressed in newer versions. If we talk only about the TM1 engine, it's quite rich and possibly too rich, making it complex to perform simple tasks. The abundance of features results in complexity, requiring strict guidelines for developers to ensure simplistic approaches are adhered to. This complexity makes it hard to maintain and evolve developments.
