Ric.ror
I think it is both - concern about rent arrears (often not all the rent is paid by housing benefit) & concern about the possible quality of the tenant & the eefect that might have on the property.
However, it has to be said there are very many people on benefits who are excellent tenants - & a lot who are not on benefits who are very poor tenants! I don't think it is fair or reasonable to stigmatise people in this general way. You need to make whatever enquiries you can about prospective tenants - including a credit reference check. Whatever agent you use to let the property should do this & you should make sure they do do it & that you see the results.