The original question included "accurately" and that is the killer for on-board systems in respect of for what the output might be useful.
Accurate weights are the true 'ideal', but consider the real world.
Commercial operations, such as mine (passenger) use standard weights for passengers and their baggage. Even the DOW of the aircraft includes an allowance for me, a figure I am still trying to aspire to! And I'm slim!
There have been a few occasions where I have been watching rotund figures bouncing towards the aircraft, with impossibly heavy carry on bags. I often wonder how the standard figures would stack up if I weighed my sponsors and their cargo.
Of course, the vast majority of passengers in a large group will average out - thus the reason we can use standard masses. But the point to be made is that we don't actually need to know to a +/-1kg degree of accuracy. On the two commercial types I have flow, both very different, we fly using figures calculated from the nearest half ton when rounded up.
Of course, the smaller the aircraft - the bigger the error even a few kg's could create.