[One (M8) Android Development] jcole20:
You don't have to flash a ruu to fix issues with multirom but a ruu is an extremely valuable tool for quickly restoring the phone to a stock state and immediately setting up your rom(s) of choice. It takes longer for the stock Rom to boot the first time than it does to flash a ruu via adb and have multirom installed. I really think users should familiarize themselves with the ruu process long before it becomes a necessity. If for nothing else, but to give them peace of mind about the safety of the whole process before it becomes an actual emergency. If you're a crack flasher I especially recommend you get familiar with the process.

Another important reason for why I regularly run a ruu when I'm trying out a lot of different roms as secondaries is even after using sdmaid to clear unneeded data/cache and removing your secondary roms and data you'll see storage shrink. What I mean is... For example, when I'm running my two favorite roms ViperOne and CM12.1 nightlys if I set the Rom and all the apps on both up properly and build all the necessary cache/data I usually have around 21gb data free on internal. After running multiple roms as a third secondary I'll see that number shrink to approx 18gb... It doesn't matter how I wipe my secondaries this happens. Yet right now I've been running cm and viper for a week now with no other additions (in multirom) I'm still at 20.8gb free internal... Not sure what the tombstones are here but this is what I see. I've said it many times I'm ocd about efficient use of storage and phone cleanliness for lack of a better explanation.

Personally I highly recommend people install secondaries to internal storage. The internal storage read/write is considerably faster than my Samsung class 10 sd. You don't have to save to internal but personally I find the best results (with regards to speed and stability) when I do so. Use your external to save your backups, music, pictures stuff like that. Use internal for rom(s), data, cache. Please... This is the most efficient way to run multirom. I stated before that flash times with sense based roms go up to around 5min where they'd normally install in 2 or 3 with internal. Boot times almost double on external. As far as speed you'll be taking a noticeable hit if you install external.

Outside of what I said above thank you for your tips... Above is just my opinion and experiences, but I really think it's an important thing to note. Especially the whole ruu process. PLEASE if you flash a lot of stuff familiarize yourself with the ruu process. I promise you that you will be grateful you've run one before if you ever get to a point where it's actually a necessity. I look at running a ruu like spring cleaning or like the yearly installation of Windows, or the clean upgrade of the new ubuntu/debian/mint etc. I run a ruu about every 7-10 Rom installations (that is if I set them up completely ... all my apps etc). Ruus are very safe and easy and I highly recommend running them via adb over hboot or via exe. Running via adb you bypass the stock rom booting if you so desire. Not to mention you see exactly where it fails if it were to. I usually wipe with fastboot oem task 28 and fastboot oem task 29 as well prior to the ruu.