To my knowledge, most mobile devices are using SD memory for non-volatile storage may that be in card or chip form. Since the Kobo is using an internal uSD card, they have control over the supplier of that memory (which means they can select suppliers that provide reliable SD memory). That leaves mechanical failure as the main disadvantage of using a uSD card. I suspect that this form of failure is relatively low, otherwise Kobo would have switched to chips long ago.

I'm actually curious as to why Kobo uses a uSD card for system memory, since very few vendors go this route. Then again, that is far from the only uncommon feature in Kobo products. They do things like provide an active serial port, they do not use a locked bootloader, and most of the software is open. (By that I mean the base operating system and many of the libraries are open source. Nickle itself is proprietary. Yet even that proprietary software is unprotected, which has enabled users to create patches.) Things like that leave me wondering if they use an internal uSD card because they want to leave their device open to modification by interested and skilled users.