The phrase "hardly useful" is correct and usable in written English.
It is commonly used to express that something is not very useful or effective. Example: While the new technology may seem impressive, it is hardly useful for everyday tasks.
They're hardly useful objects.
A camera that doesn't see all of the people in a huddle room is hardly useful.
This has the drawback that most available solutions are hardly useful in contexts different from their original setup.
These experiments demonstrate that the Agent can be optimized using fingerprint based Jaccard similarity as the objective, but making copies of the query structure is hardly useful.
In the early 1990s, as it became clear that general recommendations were hardly useful [9], the emphasis shifted from guidelines to control systems and finely tuned methodologies.
One of such situations is indoor positioning and tracking, where satellite signals are hardly useful (unless extremely large integration times are considered).
Objective Structured Clinical Examinations (OSCE), useful tools to assess bedside manners and student-patient communication, were hardly used: only one respondent incorporated LGBT aspects into an OSCE.
The Treasury secretary said a blank check would be a bazooka he might be able to keep holstered, since the threat of such a weapon might make it unneccessary to use, while anything less would be a squirt gun that would hardly be useful.
Engineers they might be, but hardly the useful kind if this EP is anything to go by.
But what may be a rhetorical necessity for presidents is hardly a useful concept for policy makers.
It would hardly be useful for Mr Morsi to visit America in the run-up to its presidential election.
