I completely agree. I won't go into it, but I went through two FitBit Aria scales and about 12 hours of setup time before calling it quits. Terrible product, even though I like the FitBit Flex.

I had a terrible time with my Fitbit scale -- finally lost the ability to sync and I said to heck with it. I now have a Withings (which I find easier to read anyhow).

Scales can vary widely depending on placement too.

I don't have an Aria but I do have a scale that estimates body fat and I think the technology is about the same. What I found... It is really easy to fool the body fat estimator. My scales weight measuring is pretty consistent in my current apartment, but in my old apartment it would be very variable because the floor had more give. So about the weight, maybe try different places? If it is on carpet it may be less stable and give varying weights.

Anyway about the body fat estimation. I always thought it was high, but I would try to only weigh in similar conditions. (For me in the morning, after using the restroom, before showering). If you drink water it can effect it, if you haven't used the bathroom it can effect it--but all that adds weight and is lean mass. Lean mass is anything that is not fat. I did feel it showed trends in my progress if conditions were similar with each measurement. It did estimate higher than my calipers or calculators where you enter in height and measurements. But then I had the opportunity to participate in a university research study and they did a bodpod measure. Bodpods is said to be quite accurate, it is used in research studies but it can be fooled in certain circumstances. My bodpod estimate was very similar to what my scale estimated that morning (it was a little higher, sadly). The bod pos was measuring full body, my scale just up one leg across the hips and down the other leg. I am quite evenly proportioned but a lot of my body fat is in my chest so it makes sense that my full body estimate would be higher than a lower body only estimate. The reverse might be true for a woman with more of a pear shape who carries little fat in her upper body. I actually trust my scale a little more than I use to now. Calipers can only pinch the fat beneath the skin, while the scales can estimate the fat deeper in the body. So it is quite likely that a scales (bodpod, underwater testing, etc) estimate would be higher than a calipers, measurement estimate for many people (maybe most).

I haven't used an Aria though. Mine is an older Tanata that cannot link to anything. When I put a weight on it, it weighs correctly if the weight is heavy enough (I think body scales often have a minimum weight they can work with). In addition to placement of the scale, how you stand can make a difference. I try to stand with excellent posture and with my weight evenly between the two feet. Then I wait a little while for it to settle. Also with the body fat scales where you put your hands can make a difference. When I stand with my hands touching my hips/thighs it will give a lower body fat percent--because then my hands are factored in. So I try to hold above waist height to be consistent. Weighing at different times a day (after you wake up) will result in different weights especially if wearing clothes vs. not wearing clothes. Other than the weight of the clothes, people can vary in weight 5 pounds or so throughout the day depending how much fluid and food is in you.