"was found challenging" is correct and usable in written English.
You can use it to describe a task or situation as being difficult. For example, "The math problem was found challenging by most students."
Exact(2)
Interpretation and utility of the MCDA value estimate (i.e., the figure 0.44) was found challenging by committee members.
Indeed, the telehealth was found challenging to use [ 31], but there were many comments on the lack of adequate training and support.
Similar(58)
Random molecular motion is also an essential factor in the explanation of diffusion, a concept that has been found challenging for students (Friedler et al., 1987; Odom, 1995).
Follicular fluid is a plasma filtrate with the concomitant large dynamic range of protein concentrations that make detection of lower abundance proteins, where new clinically useful marker proteins might be found, challenging.
Scientific argumentation was found to be challenging.
Dynamic load demand was found to be challenging to meet, requiring RFC and or battery sizes significantly larger than those required to meet average power demand.
However, the major challenge was found with the mixed patterns.
Most notably, the major challenge was found with the mixed patterns.
Globule leukocyte infiltration was found higher after the challenge 1 in resistant kids.
Those who charge such extortionate amounts should be found and challenged.
Although the formulation of multi-objective problem for water distribution network is simple, solving and arriving Pareto front is found to be challenging.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com