This is actually a good question. Before the Rosetta stone work there were a number of published "translations" of Egyptian hieroglyph texts that were honest attempts at decoding the written language (as opposed to frauds). Typically they were somewhat nonsensical and this fact was papered over with "it's a religious text". In my opinion we see the same thing with proposed translations of yet undeciphered scripts like Rongorongo. One thing these typically share is they rely heavily on proposed ideograms and have little or no phonetic component, because there's more room to massage meaning with ideograms.