How accurate are smart scales?

My understanding is they are not accurate, and can vary widely based on one's hydration level.

First of all, those things work by sending a tiny electrical current up one leg to a receiver, which measures how this signal changes. As an electrical current is lazy it takes the shortest way back to the receiver, thus down the other leg. So all it measures is the change in electrical signal in your legs and the big inbetween. Then of course there's likely an app that uses some statistical data for age, gender, weight, size, typical fat and muscle distribution for your group and a few other things, based on which it calculates your muscle and fat mass. If you're totally average then this might work for you* but if you're not it might be totally off. Anecdotal: When I stand on such a thing I get fairly high muscle and low fat measurements (all my fat sits on my legs and bum though). When using such a thing that you hold in your hands (thus the current goes through your chest) I'm given a huge fat mass (recently 41% at a BMI of just over 22!). I guess I'm not average.
* and back to the star: how hydrated you are, how much salt you have, how sweaty your feet all influece the measurement as well, even if you're totally average. Thus there's a huge margin of error, even from day to day.

Adding to the choir... just ignore all the other numbers but actual body weight.

Alternatively, you can focus on the direction the other numbers are going, but I’d still recommend ignoring the actual number. My smart scale is pretty consistent about body fat percentage, but it’s also consistently very different on that scale compared to proper body composition measurements I’ve had taken a few times, which also are consistent with each other. So, I ignore what my scale thinks my body fat percentage actually is, but since it’s slowly and steadily coming down as I lose weight I care about the overall trend is.

I might not rate them, but that doesn't stop me pouring over the data once a week to try and look for signs of improvement Tbh, the handy thing is that it auto-sends my last weeks weight readings to fitbit when I connect which is usually about once a week.