There are a lot of good CMS and web application framework choices to choose from. Yes you could build a case management system that plugs into mojoPortal and I'm sure you could do the same with Umbraco or DNN. Each has different strengths and may appeal to different people for different reasons.

I think you should do your own evaluation and decide what is best for you and your team. Making a decision based on other people's opinions may not be the best idea. Once you learn any system in depth then you will of course like it better than other systems but deciding which one to invest your time in learning in detail is probably done best by evaluating each and try to build a simple feature in each to see how the development style differs in each and you may find one system is more approachable or easier to understand for your team than another system.

Anyone who says x is better than y and is the only way to go should be taken with a grain of salt. There are many good options to choose from and each has its fans. I don't recommend choosing based on popularity.

But, definitely I recommend not building from scratch.

Best,

Joe