Not pretending being complete my humble opinion:
Mendix can be be used for high-volume web apps but it needs heavy hardware and optimization to get that working. It was not designed for that purpose. A custom written lower-level language site will win on performance but not on implementation time and costs. It depends on the requirements.
Self service can be done in Mendix.
Rich UI: you have to define that, but prepare to write widgets.
You can negotiate prices.