Besides the Salem/Beverly/Swampscott "black hole" in which WUMB and its Gloucester repeater, WNEF, are swamped on most radios by WMWM at 91.7, what are the technical factors you speak of?
There's two issues: first is that WUMB's main signal from Quincy is a (comparatively) teeny 680 watts. It really only covers inside Rt.128 and not under any adverse conditions like building penetration. Compare that to WGBH's 98,000 watts or WBUR's 12,000 watts (both from MUCH higher locations) and that makes for a pretty uneven playing field.
Second is that WUMB's repeaters are mostly on co-channel or 1st-adjacent frequencies. Even though Grady Moates has pulled some nifty tricks to synchronize everything, there's inherently large zones over Metrowest and the South Shore between WUMB and WBPR/WFPB-FM where neither signal comes in terribly well, and when they do, they tend to interfere with each other. When/If WAVM ever starts relaying WUMB, that'll help in the Metrowest area, and the new Marshfield allocation will help on the South Shore, but it's still not ideal because both those stations are on 91.7FM...1st adjacent. Even discounting the self-interference problems, the fact that's 1st adjacent inherently means you just can't pump that much signal out of those facilities, and that means a lot of in-between areas with weak signal coverage.
BTW, WNEF is up in Newburyport (Amesbury, specifically), not Gloucester.