There was an apparently significant number of JPN Bonk 2 HuCards which are defective. 5-10 years ago there were numerous brand new sealed JPN Bonk 2 on eBay. I bought one, but also already had a US copy.
The brand new one would glitch out and have garbled graphics. It was impossible to play past the first stage. These JPN Bonk 2 issues are reported from other people also.
I've also had 3 copies of Sega My Card Zoom 909 which don't work.
But overall; besides needing to occasionally clean the contacts, I have dozens of Hucards and MyCards, and none have failed besides those two mentioned.