Fish scales are as good as what you use with it. If what you use is not centered well on the device, it moves, or if there is flex, or stretch . . . . any and all of these things change the accuracy. Fish scales are best for dead weight that is well centered on the device. That is how they are designed to be used.
I used fish scales for a time, and decided against them later. Close always left doubt in my mind. If you are not concerned with precision, then they give you a reasonable idea what the bird weighs. They are cheap and accessible. I would recommend them above any other thing if precision is not the goal.
A bathroom scale is not especially useful.
The postal scales are excellent. They can accurately weigh very light weights, and the heavier weights. I can weigh a bird at 4-6 wks. or at 1 year.
The old cocker scales are great if you can get your hands on one and/or you are willing to pay what they cost.