I agree that LibreOffice may deviate significantly from OS-native UI conventions, but this is reasonable given its nature as a cross-platform project based on highly-portable OS-independent libraries. And there are plenty of applications that target particular platforms, but employ their own set of custom UI conventions; some are worse than the platform standards, some are better. It's the usability of the application's own set of conventions, and the consistency therein, that should be the basis of judgment; using the OS's stylistic defaults as the benchmark seems relatively arbitrary, especially, again, for a cross-platform application.

I don't know how it compares to iWork, but when I compare LibreOffice to Microsoft Office on the basis of internal consistency, parsimony (for lack of a better term), and even adherence to established platform conventions (on Windows), LibreOffice wins on all counts. With each new version of Office, Microsoft adds yet more UI novelties to its haphazard collection of product-specific menu styles, dialog boxes, and toolbars.