I used to mount the Memory Stick Card from my camera in a Mem Stick-to-USB adapter and drag and drop the AVCHD folder on the iPhoto icon. This didn't work with Photos. I got the "none of these files can be imported to your photo library" error that others have mentioned.
However, after a couple of drag and drop attempts I got distracted by something else and just left the Memory Stick mounted on the desktop and Photos open while I did another task. Lo and behold when I got back to Photos about ten minutes later, there were the contents of the Memory Stick ready to be selected and imported. I did a couple of other trials and found that Photos did eventually recognize a mounted card as a "device". but it took anywhere from 1 to 4 minutes, longer times for bigger cards with more content.
So, Photos can import AVCHD media in a quite straightforward way, at least with my setup*, just not via drag and drop.
*Sony HDR-TG5V camcorder
iMac (21.5-inch, Mid 2011)
OS 10.11.6 (El Capitan)
Photos 1.5
Sandisk Memory Stick Pro Duo 8GB
Sandisk MicroMate Memory Stick DUO M2 Card Reader (SDDR-108-E12M )