Sensing viscous attenuation of pressure waves is a promising principle for future state of the art fluid monitoring tools. The determining parameter in this context is the longitudinal viscosity which is a superposition of shear and dilatational viscosity. A distinctive advantage of this sensing principle, compared to many resonant-operating shear viscometers, is the large penetration depth of pressure waves in fluids which enables probing the bulk rather than a thin surface layer. In 2011, we presented the first prototype of such a longitudinal viscometer featuring a cuboid-shaped chamber with a sample fluid. One boundary surface carried a flush-mounted PZT disk operating as pressure wave transducer. The basic sensing mechanism of the device exploited the damping of ultrasonic, resonant pressure waves in the fluid in order to measure the longitudinal viscosity. Since then, numerous sensor variations featuring different device embodiments (e.g., a transmitter combined with a wedge reflector, a transmitter-receiver transmission line, two different types of tube resonators etc), functional principles (e.g., resonant or pulsed operation), and viscosity value determination methods have been investigated and published. In this contribution, we validate assets and drawbacks of these sensor variations regarding functional principle (e.g., spurious mode reduction), modeling approaches (e.g., diffraction loss modeling), and measurement methods (e.g., efficiency, stability, and reliability). Finally, we compare the performance of our longitudinal viscometers to well-established resonant shear viscometers.