i had chickenpox as a kid, i remember the aveeno baths for it, we were set in the same room to "inonculate" the rest of the siblings. as there was no vaccine at the time. Chickenpox is quite severe for adults though. i did get shingles around 20yo though. theres is shingles to potentially turn severe, but its rare. shingles can cause meningitis, and encephalitis, as well as spinal cord damage.
people who arnt sure about thier chickenpox immunity can ask thier doctors to do antibody titers(it doesnt detect dormant chickenpox in your ganglia though because theres no way to detect it outside of autopsy), your doctor maybe reluctant to administer the test though.