The phrase "are often doubtful" is correct and usable in written English.
It can be used when discussing situations or people that frequently exhibit uncertainty or skepticism.
Example: "Many experts in the field are often doubtful about the effectiveness of the new policy changes."
Alternatives: "frequently skeptical" or "often uncertain".
This is due to the paucity of diagnostic characters and the fact that these characters are often doubtful to score and interpret, particularly when relying on traditional light microscopy [5].
In particular, physicians are often doubtful of performance of such systems because their application in clinical practice requires a careful assessment in a daily routine fashion.
Due to currently unavailable quantification methods, the measurement of structural constituents on sections is usually difficult and the results are often doubtful [ 24].
However, the effectiveness of consolidation of calcareous materials is often doubtful, due to the lack of effective and compatible consolidation products.
Every writer who is original is often doubtful about the value of a work; praise from a critic whom he respects is a treasured reassurance, silence or blame a confirmation of his worst fears: "So I'm found out and that odious rice pudding of a book is what I thought it — a dank failure".
The number of graduates is small relative to the size of the population about 10% of Indians receive any college training, as opposed to 50% in the United States and the quality of their education is often doubtful.
At their first clinic, patients' attitudes were often doubtful regarding the reality of their risk of developing T2D within 3 years.
Upon learning that he is gay, the people who already know him are often surprised or even doubtful that that could be true, because they have seen how gender-conforming he is.
One should not be deluded by high bootstrap or posterior probability values, because high support for a doubtful clade is often obtained when a very large number of characters (like whole chloroplast genome sequences) but small number of taxa are used in phylogenetic analysis.
The specific evidence cited in his books was often doubtful, Professor Burstein added, but "he succeeded in putting the question of the origins of Greek civilization back on the table".
