Except that the Nexus 6P came in 32/64/128 GB configurations and the Nexus 5P came in 16/32 GB configurations.
They also don't allow their apps to be moved to SD, which is especially aggravating for the crappy apps you can't uninstall on phones with small built in drives.
FYI at least Samsung seem to be realizing their mistake as their latest models have the microSD card slot again after having abandoned it for some time, so it does seem like it's back.
I also have to concur with what others have said here - I've done my fair share of Raspberry Pi and ODROID-XU4 system image tinkering and come to the conclusion that microSD cards suck big time as a general purpose read/write storage device. I wouldn't feel comfortable having my phone's internal storage running off one actually.
Now if phones came with eMMC slots.... :)