The handful of applications that are functional and potentially useful tend to be incomplete or suffer from usability issues.

This seems like a major usability issue.

The user interface has been refined, and many usability issues have been addressed.

These types of studies are useful for validating suspected usability issues.

Though seemingly unsophisticated, this method is very successful at discovering usability issues early in the design process.

It does have some usability issues, though, and these have been retained.

This layout allows using keyboards of the same physical design as in many other countries but has some usability issues for Russian-language users.

You're right, that is totally a usability issue.

This allows designers to identify any usability issues that may arise in the user interface before it is put into wide use.

These examples provide only a brief sample of the usability issues of the software.