Wow, that's a pretty awful UI. The default search form screams "advanced search" from the late 90ies. The compare example looks pretty dated too. I think Kibana and Sourcegraph are on the right track with a single input field that accepts field:value type searches. They're great once you've learned them.
Opengrok implements hyperlinking for C/C++ code which is the primary productivity multiplier since it allows you to easily jump around callgraphs. That functionality is sorely missing here (unless i am missing this feature somehow).