The phrase "was rather challenging" is correct and usable in written English.
You can use it to describe a situation where something was difficult to accomplish. For example, "Completing the assignment was rather challenging, but I eventually managed to do it."
"Fiscal 2001 was rather challenging for Lucent, so the grants were made to ensure Henry had management stability through the turnaround," said Mary Lou Ambrus, a Lucent spokeswoman, in explanation.
It was rather challenging at first but we figured out an easy process.
Even the provision of HBV vaccine within 12 hours of delivery was rather challenging.
Also, it was rather challenging to get parents to accompany older minors to the clinics for the consent process.
It would have been optimal to do so, except for the fact that, logistically, it was rather challenging and would have incurred the extra cost of videotaping.
The control design was rather challenging because the system is highly nonlinear and because undesired interaction occurs between multiple illumination zones in a big office room.
Unfortunately, this was a great idea in theory but running an entrepreneurial business within the confines and culture of a bank proved to be rather challenging.
My Tokyo colleague tells me that this is because there is so little focus in the Japanese media on the decades ahead and, as Megachange argues, those decades could be rather challenging for a society ageing faster than any other in history:By 2050 the country will have almost as many dependants as working-age adults.
However, themeasurement techniques are rather challenging.
However, precise control of these geometric parameters is rather challenging in practice.
Therefore, compared with general object recognition, this task is rather challenging.
