The problem is that refractometers were designed for simple sugars. The complex sugars in wort make them inaccurate. That is, the brix reading is incorrect.
I think the error is estimated at about 4%. However, it depends on the wort. A heavy wort with lots of complex sugars will make it even more inaccurate, and a highly fermentable wort will have less error.
Because all worts are have different mixes of sugar, you cannot get one that will give an accurate measurement in beer wort. They may (or may not) make one with the 4% factored in, however due to the different make up of every wort, it will still be slightly inaccurate.
It is quick and easy tool to get a close estimate of OG. Just keep a copy of the conversion chart handy. For measuring (and of course calculating), gravity of fermented wort, I don't trust it at all and will always use an hydrometer ........ I can drink the sample too!