Built in memory vs SD card.

I've been asked about mobile phones (not my strong point) specifically about the memory they have.
One can purchase a phone with say 64GB of memory for a lot of money or one can buy a phone with little internal memory and a high performance (UHS II) 64GB card for about one third of the price.

The following link says that android 6 OP system will have a setting to 'extend' the internal memory to the SD card (I'm not talking about rooting the phone):
http://www.howtogeek.com/114667/how-to-install-android-apps-to-the-sd-card-by-default-move-almost-any-app-to-the-sd-card/

It also says that apps can only be installed to the SD card if the developers allow it.
I'm a little confused by all of this so I've got a few interlinked questions.

Is this option part of android 6 on all phones or do some makers disable this option?
Apart from the SD card now being linked to the phone is there any other downside to doing this?
Can I still have the situation of eg. 60GB free memory but an app won't install because there isn't enough (built in) memory available?
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AndyAinscowFreelance programmer / ConsultantAsked:
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Shaun VermaakConnect With a Mentor Technical Specialist/DeveloperCommented:
Is this option part of android 6 on all phones or do some makers disable this option?
It is specific to the App, not the phone. Quite a lot of application can either not be moved to SD -OR- can only be partially moved
Apart from the SD card now being linked to the phone is there any other downside to doing this?
SD Card are not as robust as internal memory and cheaper SD cards can fail without warning. I would not rely on them without a backup to Cloud etc.
Can I still have the situation of eg. 60GB free memory but an app won't install because there isn't enough (built in) memory available?
Yes. If Phone is full, you will not be able to install Apps. Move to SD happens after the fact
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akbCommented:
As Shaun V said, SD cards are not as robust but one advantage of them is that if the phone dies you can take out the memory card and restore it elsewhere - thinking things like photos.
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AndyAinscowFreelance programmer / ConsultantAuthor Commented:
Thanks.  That has made things a bit clearer in my mind.
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