Students with more resources can pay for sites/tutors to complete assessments for them.
Is your assessment accurate?
There area number of factors that can affect your assessment. Students' stress levels, access to technology/resources, second-language issues, etc.
Is proctoring inequitable?
In general, high bandwidth activities can create equity issues. So it's really important to ensure that your class does not rely exclusively on high bandwidth activities. The blog below has great ideas of how to use both kinds of activities in your course.