Not just performance. Reliability and data loss are also bad. All of these adapters do raid-0, which means any one card failure (very common, I have seen it a lot in my personal devices) will damage the volume.
As I have a lot of odd SD cards unused in drawers I would be more interested in these adapters if they exposed every card as its own drive so I could do ZFS, or if they did "real" raid.
I was thinking it would be cool as a small NAS with a mirror. I want to put my movies and photos on it, and having a huge mirror is very useful for that type of workload. I could drive it with a cheap ARM device, which should allow me to stream a 4k video over the network, provided my network is the bottleneck.
Why not just use an actual 2.5" SSD instead of a bunch of SD cards in a terrible quality adapter?
At least with the SSD, you only have one microcontroller doing god-knows-what to hide the horrific reality of flash storage, vs 10 SD card microcontrollers and one on the adapter board, which are all probably even worse.
As I have a lot of odd SD cards unused in drawers I would be more interested in these adapters if they exposed every card as its own drive so I could do ZFS, or if they did "real" raid.