Demings’ tenure as police chief is also a problem, though, as her handling of police brutality has been widely criticized.
Progressive Democrats will be especially wary about her ability to lead a national police reform campaign given her record of deafening her own officers in cases brought against them of police brutality. The worry is that as a former police chief she will put off would-be voters.
Secondly, unlike most of Biden’s other candidates for VP such as Elizabeth Warren and Kamala Harris, Demings has not had a significant impact in congress beyond her role in Trump’s impeachment trial. 
She has not been the face of any major legislation, so may struggle to offer much beyond her policing background. This highlights another issue of picking Demings for VP: she does not have the relevant political experience that Biden wants and needs. Biden wants someone who is ready to go on Day One of his Presidency, and Demings has not proven her ability to do so.