[Samsung Galaxy S 5 Unified Development] jdrch:
Congratulations on inquiring about the best and most underrated, criminally ignored Android feature since 6.0. It's excellent; I've been using it since CM 13.0. A few points (from my own experience:)

1) Ensure the data on the adopted card you care about keeping is backed up elsewhere, or you run the risk of losing it if something goes wrong because adopted cards can be read only by the device that adopted them. Also, because of this, you'll have to transfer files to/from your phone over the network. I recommend Resilio Sync.

2) Remove the card before booting into recovery and replace it only after rebooting back into system. Doing otherwise may result in the card becoming corrupted (this bug may have been resolved, but I've never seen it acknowledged by LOS or TWRP devs and I don't consider risking it to be worth the possible data loss.)

3) Android rechecks the card on reboot and if the card is inserted after Android is already running. During this check the card is unavailable to installed apps, so just let the check complete before doing anything.

4) Only the card OR internal storage is available to (non-root) apps after adoption. Initially only internal storage will be available. If you enable data migration, user accessible folders (e.g. DCIM, Downloads, etc.) - but NOT your /data partition - on internal storage will be moved to the adopted card and the internal storage will be reserved for apps and the OS in general. You can still write to and manipulate internal storage using apps with root authority, but apps without it will be able to see the adopted card only. Despite this, some apps such as ES File Explorer store some settings on the adopted card instead of in /data & may be wonky if the card isn't available to them.

5) Personally, I recommend installing as many apps as you can to internal storage and reserving the SD card for user files. That way, a nandroid backup will pick up all your apps because they'll be in the internal /data partition.

6) As far as I'm aware the adopted card is tied to the /data partition. Ergo, an adopted card can be read only by the /data partition it was adopted by. However as long as you have a working nandroid backup on hand you'll pretty much never have to factory reset as you can just revert to an older build until the issue is fixed by the LOS devs or reflash Gapps if necessary.

7) Unlike with internal storage, the OS doesn't warn you when you're running out of space on adopted storage; you'll just notice things going wrong (podcasts failing to download, etc.) As such, it's best to get the biggest card available, regardless of cost (trust me, it's worth it in the long run.)

8) Unlike internal storage, there are absolutely no read or write restrictions on adopted storage, which means every app on your device can read and write to every folder on the card. This is by far the biggest advantage of adoptable storage as it turns a 16GB S5 into a 200+ GB PC-equivalent with a sufficiently large card.

Assuming you follow the above points, pretty much zero. As long as you have a nandroid backup on hand you can restore your phone to its previously working state.

Generally speaking, if adoptable storage chokes on you 1 of the following is the case:

8) You have a bad card. I haven't experienced a card failure in either of my 2 adoptable storage devices since I started using the feature in January 2016. To reduce the likelihood, stick to the highest performance Samsung or Sandisk cards. You really do get what you pay for with storage.

9) You're running out of space on the card (see #7 above.)

10) You exposed the card to a recovery environment (see #2 above.)

11) You may just need to remove and reinsert the card or turn off the device, remove the card, reboot, and then reinsert it.

1 more thing: because of the above, adoptable storage requires a lot more active thought (read: make sure you know what you're doing and mentally check your steps before doing it) than plain old external storage. If your files are properly backed up (again, Resilio Sync is strongly recommended) and you have a nandroid backup then data loss is only a temporary annoyance (but an annoyance nonetheless. Re-syncing 120+ GB of data takes a while) but if you don't you'll lose everything on the card. Your /data partition on internal storage will be fine, but any files and apps on the adopted card will be toast. Just bear that in mind.

Also, adoption exists at the filesystem level only, so wiping the card on a PC or on a phone will make it useful for other purposes like any other card, with the caveat that you'll lose everything on the card that isn't backed up somewhere.