Smart scales do two very different jobs, and they are not equally good at both. Weighing you is the easy part, and any decent scale gets that right. The body fat, muscle and water numbers are where the trouble starts.
Those numbers come from bioelectrical impedance. The scale sends a small current up one leg, across the pelvis and down the other, and measures how much the body resists it. Lean tissue holds a lot of water and conducts well. Fat conducts poorly. The scale then feeds that resistance, along with the height, age and sex you typed in, into a formula.
That formula is the weak point. Each manufacturer uses its own prediction equations, usually built on a particular group of people, and they can be off for anyone who does not look like that group. Published research comparing impedance devices with DEXA scans, the standard reference, has found they tend to misjudge individuals even when they look reasonable across a whole group. One study found impedance read body fat about 4.4 points lower than DEXA on average.
Hydration moves the numbers too. Because the current travels through water, being dehydrated, having just eaten, having just exercised or having had a drink the night before can all shift the body fat reading by more than a real week of change. A foot-only scale also mostly measures your lower body and guesses at the rest. Scales that add handles for a four point reading do better.
None of this makes the scales useless. It makes them trend tools. If you weigh in at the same time every day, under the same conditions, the direction of the line over a month means something even when any single number does not.
What I would not do is pay extra for a long list of metrics. Visceral fat, bone mass and metabolic age are all derived from the same impedance reading, so they carry the same errors dressed up as extra precision.
The short version: trust the weight, treat body fat as a rough trend, and measure first thing in the morning under the same conditions every time. If you need an accurate body fat number for medical reasons, get a DEXA scan instead.
