Even my rather trivial undergraduate UML project required very advanced options, and the only software that fully satisfied my needs was astah professional. I struggled a lot with browser-based UML tools, always to come across a feature I needed but wasn't supported. It was very frustrating, because I had to restart work on several platform, until I gave up and started using astah.

For this reason, I personally avoid web-based UML tools ever since. But, there might be reasonable use cases for them.