Adaptive internal storage : analysis What are the possible difficulties occur if I use external SD card using Format as internal storage ? I have checked this link:
How does the &quot;Format as internal storage&quot; feature work in Android 6.0+?

Here are some doubts: 


Does internal storage space increase, if no why?
Whether any memory problems with TWRP, Titanium backup?
What to should I do, If I need to switch with another SD card as external OR restore to earlier?
What will happen, If I accidentally boot device without SD card?
What to do after upgrade? (upgrading to higher android versions OR to lower android versions )
Will this action improves performance?
The internal storage doesn't increase itself, but external storage start behaving like internal (you can move apps to it and so), so you can say you have more internal which is used for apps mostly
With titanium backup you shouldn't have any problem as with other apps, for TWRP I can't say anything because I didn't have that experience, but since adopting is android feature recovery shouldn't make you a troubles
If you pull out SD card which is adopted app which is on SD card (adopted storage) won't function for obvious reasons, and SD card can't be used with other phone because of encryption. If you put other SD card instead of one you have adopted new card will behave normal, like external storage.
Nothing special, just apps which were on adopted storage (SD card) won't work, and instead of their icons there will be little green android, also data stored on adopted storage won't be there (obvious).
Don't know about this one, but as I said adopting is android feature so there shouldn't be any problems, but as we know sometimes after upgrading phone goes little crazy, so there is chance you maybe should perform factory reset, but just maybe.
If on performance you think on faster loading apps, that won't happen, SD card are way slower than actual internal storage, but there won't be much difference in loading time maybe for some big, very big games/apps, basically, most of people who adopted their SD cards done that because they needed more space for apps
So if you want to adopt SD card make sure that is at least class 10 that would be minimum, and speaking from my experience adopting could kill your SD card not really kill it but kill r/w speeds so it becomes 10x slower, happened to me with 16 GB transcend, also it is possible that SD card was crap.