an electric impedance scale can be just fine. Every method has it's error and though impedance "can" be very inaccurate, they can also be very quick, easy and accurate and usually are. The most accurate ways are also the most impractical and expensive.
They key to any of the more practical ways to determine body fat % is to take the measurements at the same time every day and do it multiple times. Example, 10 am, I step on the scale 3 times, take my average and do it again the next day. Getting a "data point" takes about 30 seconds or less and let's you know if your scale is all over the place or not.
and in the end, they're just numbers. If you're 25% or really 20% at any one singular point in time, does it really matter? It gives you a point and you make food and exercise choices from there and in 2 -3 months you check it again to see if what you are doing is working. Are you moving in the right direction? and most of those scales are just fine for that.
Getting an accurate body fat percentage isn't like you're diagnosing cancer or an aneurysm or something. being close enough is ok.
And if it says you're staying at 20%, but you're really 17 or 26%, and you're eating healthy and moving quite a bit, who cares, they're just numbers. You aren't going to win a prize and if you're doing all the healthy things you can do, you're as healthy as you're going to be. And if you're 26%, but you want it to say 22% and you're already doing everything as "healthy" as you can, what are you going to do differently to change those numbers that's going to make you more "healthy"?
Anyway, we use impedance mostly because it's easy and accurate enough. I think Dexa is pretty impractical and very expensive for equipment and like I said, we're not diagnosing a brain aneurysm. It's body fat. Plus why the need for any amount of ionizing radiation for body fat measurements?
Hydrostatic weighing? accurate and incredibly cumbersome. Calipers are just fine if the person doing the measuring is good. But i've found most people don't like exposing more skin and being pulled at and pinched. But it can be a very economical and efficient way to get a body fat measurement.