"For example in the 19th century linguists were able to deduce that the Proto-Indo-European language must have had guttural consonants not found in any extant language, and then later when the Hittite language was decoded, the guttural consonants were right there. The theory was validated on held-out data."

I find reconstruction fascinating, but it will never be completely accurate, because it just can't be. Every language has quirks, and I believe PIE probably had one or two complex features that never survived into the age of writing. Most of its vocabulary is lost, although I hold out more hopes for phonology.