My main cameras have two card slots.  One is a CFast and the other is Compact Flash.  I use the CFast all the time but the Compact Flash is a handy backup.  Occasionally, if I have the camera on with the CFast out of the slot, the camera reverts to the second slot and, if I don’t notice, it continues to use it when I next shoot.  This isn’t a particular problem except when it comes to downloading.  I have USB3 card readers for both CFast and Compact Flash.  However, the speed of card technology has moved on dramatically.  When I download the Compact Flash cards and import to Lightroom, I am reminded of just how slow they are.  I used to do this all the time but, once I started using CFast, I got used to the better speed and now, when I revert to the old tech, it feels positively glacial!