Hyperion SMGs are highly accurate but they have the feature of "reverse recoil": the weapon has initial sway and only becomes accurate with sustained shooting.. The listed accuracy on the weapon is what is achievable after the sustained fire increases their accuracy to maximum.
The Weisenheimer is part of a sub-group of SMGs that only can spawn with a fixed-element Maliwan barrel. They feature the higher-than-usual elemental damage/second and element chance granted by the Maliwan barrel. These parameters achieve even higher values, if the SMG is equipped with a Maliwan grip.
Despite the low rarity designation of these weapons, their high elemental chance and status effect damage can make them excellent choices for combat until a more suitable (or less common, e.g. Legendary) weapon is obtained.