Not all digital refractometers are bad. I purchased a Jemeter Digital 90. Like anything else it takes practice with your equipment. You can get hi low readings by slightly moving the gem over the eye. I take 4 readings per stone, and I use the highest and lowest numbers.
I still think the manual method yields more accurate result more consistently. It just takes longer. There is a reason the method hasn’t changed. With the Jemeter I get readings that are off by small amounts, because the gem must be completely clean. Any, and I mean any, residue will throw results. You will also get crazy results if the gem is not large enough or if it’s not faceted completely flat. Light escaping edges is an issue since air has its own RI 1.000277, and you can’t can’t calculate that for that in your results.
Remember the refractometer is just one method on your check list. I suggest you question the advertising on digital models. They most likely have stones the know will show the perfect number. Calibration can be hard.