Does it make sense to buy usb 3.0 sd card readers?
I have two external USB 3 hubs, each connected by a USB 3 cable to the USB 3 socket in different PCs. The hubs are different makes, though could contain the same chip manufacturer's hub IC.

If I plug a USB 3 flash stick into the USB 3 hub, I can write to it at USB 3 speeds nicely. 

If I plug a USB 2 flash stick or USB 2 memory card adaptor into the USB 3 hub, the speed of writing to my still-plugged-in USB 3 flash stick drops to USB 2 speeds. So the hub appears to drop to the speed of the slowest device.

I can see the USB-protocol reasons why this happens but it isn't how USB 2 FS (12 Mbps) devices and HS (480 Mbps) devices coexisted.

So, for me, there is a good case for only using USB 3 devices for these PCs, saves the unplugging and replugging, so long as the cost allows.

Incidentally, although it might appear as a serial to serial converter from the ends of its cables, it's taking a block of data from USB, buffering-and-checking it, then sending that block of data to the target flash card. As I imagine you already knew, communicating over USB bus is a separate activity to communicating with the target card.