The phrase "are useful only when" is correct and usable in written English.
It can be used to specify conditions under which something is beneficial or effective.
Example: "These tools are useful only when applied in the right context to achieve the desired results."
Alternatives: "are beneficial solely when" or "are effective only in cases where".
In conclusion, autocalibrated systems are useful only when used to monitor changes in SV during fluid challenge in stable conditions but become less accurate with changes in vascular tone and reactivity.
For the most part, recipes are useful only when you already have a pretty good idea of how to cook.
The views of readers are useful only when supporting the paper's current line on any controversial issue.
Chat Heads are fun and effective, but Facebook's engineers appear to have overlooked one small detail: Chat Heads are useful only when you receive a message.
He appeared yesterday to sympathise not scowl when things went wrong; to appreciate, at 30, that talent and self-expression are useful only when it suits the team.
Because search engines are useful only when there is an abundance of researched and reliable material, the company has paid translators and offered Google translation kits to foster content in many languages that are underrepresented on the Internet, including Arabic and many in South Asia.
(We'll leave "financing" out of the picture at this time, because despite how badly you need it, it will be useful only when you use your business plan to get a good job and move up the ladder).
Dr. Carmen Simon, in her book, Impossible to Ignore, says that the past is useful only when it helps us to predict the future.
As mentioned in Section 3, interlaced BI-RL is useful only when pixel-wise computations (2) and (5), instead of FFT-based ones, are used for ({mathcal T}) and ({mathcal T}^).
Ongoing efforts to predict hot spots of infection on both small and large scales is useful only when at-risk communities are able to use the information to target mosquito or vaccine control efforts and prevent outbreaks.
DNA testing is useful only when there is physical evidence to be tested.
