From my experience adopted storage doesn't work, and is very evil. After only 4 days my Motorola is is in hospital. For what would cost only pennies, my phone is very ill. 8gb on Lollipop or Kitkat was plenty, but on Marshmallow it's nothing
I have seen your posts a lot complaining about this device and how terrible it is... The problem is this was your experience, and 98% of all other users don't/won't experience a fraction of the issues you have had, much less all of them. Or course, it is possible your device was just a lemon.
Adopted storage works, it's not great but if properly adopted and migrated to a high speed, quality SD card it works very well for most users.
From what I've seen of your posts, the migration process did not occur or didn't complete properly, and given a reasonable time period and assistance from the community here the problems could likely have been corrected. Instead you chose to send it off, which is fine, but you don't need to badmouth a quality product... There is no questioning that fact, the Moto G 2015 is one of the best rated devices under $400 since the inception of Android, every single reputable Android site (Android Police, Android Authority, Android Central, etc) will back that up.
And as for Marshmallow, it really doesn't require anymore space or resources than Lollipop or KitKat.