Adult cases are not fatal by a large majority. The highest number I could find said that adults were 25 times more likely to die from chickenpox than children.

I found a death rate of 21 per 100000 adult cases. That's not a majority. [0]

Also, it's better to never get chickenpox at all, as shingles is very painful. Shingles is the reactivation of the chickenpox virus. If you've ever had chickenpox, you are at risk of shingles.

[0]: https://www.cdc.gov/vaccines/pubs/pinkbook/varicella.html