Hypotheticals don't help with his question, either.
He asked if he should use a flash drive instead of a HDD. The answer is no, it's not worth the trouble of finding one that may work only to fail randomly in a few days.
If you use GOOD QUALITY media, you're fine. Flash memory is designed to be used. It's not meant to be a paperweight.
If you use bad flash media from eBay, you're not fine.
The problem is with the media. Not the Wii "killing" it.
Edit:
This is how I envision it goes:
Timmy goes on eBay, sees a 256gb USB for $20 shipped, when they sell in-store for something like $80 + tax.
"Oh wow, what a deal! Let's order this!"
A couple weeks later the USB arrives. He's like a kid on Christmas when it comes, he just can't believe the deal he got! It was a steal!!
He sets up all of his pirated ISOs and games on the USB. Connects it to the Wii. What a dream!
Then, days later, it fails. He goes on the forums to ask "what happened to my brand new USB? It was working and I used it on my Wii and now it's dead!"
The conclusion: "Wiis are killing flash drives!! They are clearly incompatible! I mean, just look at what happened to this guy's USB drive! It was working fine, and now it's not!!"
Reality: the USB drive was a piece of junk destined to fail.
Moral of the story: buy good quality memory from the store.