Do they say that? If you’re hearing that, it’s not from people who drink kava and know kava. The majority of the science that’s been done on kava confirms that it’s safe. Beyond that, the real-world experiment has been running for centuries. Melanesians have been drinking kava daily for a very long time...the Polynesians too, perhaps to a slightly lesser degree. There doesn’t appear to be much, or any data, scientific or anecdotal that would support the idea that regular kava consumption is uniquely dangerous.
That being said, genetic outliers exist and if a person happens to be a genetic outlier in the ‘wrong’ way...then kava (or other substances) could be unsafe to consume regularly.