Hi Jens,
In my experience, the web interface has the following drawbacks:
1) Not possible to create or modify charts
2) Not possible to create or modify dashboards
3) Difficult (but possible) to create complex queries
4) Very difficult (but possible) to do anything complex in Source
5) Poor error handling compared to the client (if your network drops out, you've pretty much lost anything unsaved... the rich client will either reconnect automatically or at least allow you to try to save it again later)
With that said, the web interface does have an advantage over the rich client in that the web interface does not use tons of RAM on the user's machine.
I have not used the SOAP APIs much, but I imagine that most of the basic activities are supported, but the more complex activities are either very difficult or impossible to do, just like with the CLI and Java API.
Hope that helps,
Matt
