If you look at the armour totals in the vehicle armor spreadsheet then you will be able to see that the harasser does have the lower armor values of pretty much anything in the mod currently (comparable to Osiris's), and as with any vehicle, it does take extra damage when shot in the side/rear armour.
It is however, a dangerous asset for starting mechs, considering its speed and firepower, it can be quite tough to get to grips with when used by a skilled opponent. All I can say is, focus them down fast, otherwise their fairly adequate weapon loadouts may end up having quite a surprising effect on your starting mech.
Armour and hull strengths of hovercraft were dropped since last patch, so you will notice for example, that the heph is considerably weaker than it was last version.