There was quite a bit of discussion regarding regulation of turbofan/turbojet shaft speeds. However a major consideration with turboshaft engines is regulating torque in the output/power turbine shaft. Most turboshaft engines can easily produce greater levels of torque in the output/power turbine shaft than it can safely handle.
The problem of shaft torque capacity can be quite significant in high power engines that have a forward PTO and the power turbine located aft. The PT shaft of these engines ends up having a modest diameter and long length. Many turboshaft engines (like the PT6) locate the PTO at the turbine outlet to avoid this problem.
Older turboshaft engine torquemeters often measured fluid pressure produced in a hydrostatic device attached to some part of the output drive that displaced in response to torque inputs. Newer turboshaft engine torquemeters typically use electronic sensors to measure torsional deflection in a calibrated output shaft section.