I have a number of rentals in Hagerstown. The number one requirement to being moderately successful there is ignoring your vacancy and being very strict with screening requirements. You must get access to the Corelogic eviction database and screen out all prior evictees in that market. Maryland evictions do not appear on case search, Corelogic sends reps into the courts to enter the records manually, searches are around $8 per applicant.
Hagerstown is a lot of work. The city is against you. They will fine you when weeds pop out of the sidewalk and fine you when your tenant needs police assistance. Property taxes or license fees increase annually. For a lot of headaches my rentals there return 4-6%. I've stopped buying there and now focus primarily on Frederick. It's just too much work for the return.
If you're planning on landlording with any debt on your portfolio, forget Hagerstown. You will be cash flow negative on every deal with financing attached to it - this is almost a guarantee.