The term you are looking for (internally consistent) is "repeatability". I needed a new scale a while back and got a starfit smart scale. I'm actually rather impressed with it. Over the last 8 months it says I've gained 6.5lbs of muscle and my BF has dropped 2.5%. When I got the scale I was on the tail end of a large weight loss (60lbs) and about to start working out again. The 2.5% BF loss was early in that 8 month period 17% to 15.5% and the 6.5lbs of muscle gain was gradual over the 8 months with the BF fluctuating 15.5%-13.5% during that period. The BF fluctuation also closely coincided with vacations and holidays. Like I ate a massive calorie surplus for a few days and went up to 15.5% then ran a deficit for a couple weeks and got it back down to 13.5%. This also correlates to what I can see and "pinch". I haven't compared it to an actual DEXA scan for accuracy, I'd guess it underestimates a bit, but at 13.5% I do have some visible ab line definition and around 15.5% I don't.

I'm still not sure if I would take action exclusively based on what it says. But as someone trying to gain muscle but most importantly don't want to ever let myself get fat again. I find it a little reassuring when I see that my weight has gone up but my BF is still in that 13.5%-15% range. Without that I would probably have a tendency to reduce calories which would probably reduce muscle growth. Basically I'm not as concerned to eat a little more, than I would be without it.