I havevone cheap refractometer. I struggle with it. If I calibrate it properly and get my test sample to the proper temp, it works. I dont trust it but its good for things I cant practically use a hydrometer for, like sugar content of juices.
My hydrometer is the cheap variety and works fine. Occasionally I check the calibration in water. With mine ive noticed the paper measurement will slide if its bumped, and you can slide it back with very gentle tapping one way or the other. I rarely have to adjust because im pretty careful with it, but when I first got it it was about 6 or 8 points out of wack.