Flex is great, but as a consumer you have to educate yourself on some of the nuances. Giving time for the car to figure out what's in it on fuel changes and monitoring that "somehow". I use the the AFR gauge personally since it'll show what the car thinks is in the tank and I'll know when it's figured it out. Mine usually does it within 5 minute or so, so like Jon says don't just pull out of the gas station and go WOT. Mine's a 2016 model so the FF stuff didn't give me too many problems, but different OS react differently as Jon also points out, so keep that in mind. Some just flat won't work from what I hear as well, so changing OS versions is the solution there. Not a huge deal at all though, just need someone who can do it. OEMs change OS versions on in the field vehicles at times so don't be worried about that.
Going back and forth 5 gallons at a time also isn't recommended. Run one fuel most of the time, and when you wanna change from say E85 to 93 then run the car to almost empty and fillup with the new fuel you plan to run.
Reviewing Flex Fuel information from the owner's manual of an OEM Flex Fuel vehicle will pretty much say these same things.