Over the years a particular kava you purchase can vary subtly or drastically. If you buy a single source cultivar, that continues to be from the same source through the years, the variances won't be as extreme from year to year. Even in that case, more noticeable differences may emerge. Just as an example, GHK may sell a particular cultivar, like Mo'i, but not all of his Mo'i plants have necessarily been cloned from the same plant. Many will be clones of the same plant, but he may have acquired Mo'i cuttings from different sources over the years and those plants could differ a bit, psychoactively. So, the others, while still being "Mo'i" and sold as "Mo'i" and morphologically appearing to be "Mo'i" won't necessarily have the exact same effect as "the one" you had "that time". But it will still likely be in the general vibe.
Mass produced kava blends and the most popular cultivars that come from big wholesale operations have a greater chance of changing more significantly. They are selling so much, that they have to source from many different locations. And that can lead to batches being different. Even in this case things can stay pretty steady, but things like the rising popularity of kava and cyclone damage to crops can hinder the usual flow of kava and require wholesalers to suddenly find new sources.
On top of all that, kava can be affected by the amount of sun it got, the health of the soil it was is, the amount of rain it had, the way it was dried, the way it was processed etc... all can change the resulting beverage.
TL;DR - Old reviews are a good general guideline, and can even be valid for a few years sometimes, but they are most accurate when describing a current batch.
