The phrase "are usually suspicious" is correct and usable in written English.
It can be used to describe a general tendency or behavior of a group or individuals who often exhibit doubt or mistrust towards something or someone.
Example: "People who have been misled in the past are usually suspicious of new information presented to them."
Alternatives: "tend to be wary" or "are often distrustful."
But I confess that, research aside, I'm usually suspicious of arguments of the form "aid only hurts the people it tries to help".
I am usually suspicious of memoirs produced by writers who have not yet reached middle age, but Kazin, like so many of his provenance and generation, seemed to possess a voice of centenarian authority as a young man.
We see bamboo here and there on CrunchGear (the Dodocase, laptops, bikes), but I'm usually suspicious of bamboo-type products, because they usually make specious claims of greenness.
I'm usually suspicious of music writers who become very publicly grief stricken by the death of a musician just because they happened to have interviewed them once or twice back in the day.
Reports filed by banks and brokers are usually about suspicious transactions by customers rather than in their own accounts.. Most hedge funds operate differently because they gather funds from investors and then make the decision about where to put the money, so that a suspicious activity report is likely to involve disclosing their own potential misconduct.
Men are usually not very suspicious of women's motives, so it should be relatively easy to convince him that he is in control and that he is the one seducing you.
Others abhor change because they prefer a set routine – these are usually the people who become suspicious of change and are more likely to resist.
Since approximately 20% of women with suspicious mammography are usually found to have BC and about 80% have BBD, additional sera from cases of DCIS and IDC of the breast were also obtained from the Tissue Procuring Facility of HFHS.
