As someone who has played in and hosted such games.......... they suck.
I can say nothing on EXPERTS sans "moles everywhere" and "ununited is the worst". RotS' moles were all killed veeeeeeery quickly so they had little to no impact on anything.
The problem with ununited multifactions is, even if you give factions good players, or strong roles, or the weakest mole(s) to infiltrate, if a faction is simply inactive, or their clutch player doesn't do a whole lot, they're kind of screwed. (actually not saying anything about LW and his naggy faction rn)
Ununited becomes a large chore for whatever faction has problems assembling - if all other factors are equal and someone goes hunting early on for their team and is very active, that faction will get pulled together and start doing work a lot sooner than any other, and the ununited faction will likely take ages to catch up, or, more than likely, NEVER come close to winning.
tl;dr inactivity in ununited games literally kills teams' chances, or allows the moles to spread like a wildfire and dictate things way more than the intent was. If one person on the entire team does not claim and talk to the team, the team is in dire straights.
Other M is a perfect example of where there's just stupid crap going on - the Galactic Feds were completely unrelated to the 3 (multi)factions and it's just ?_? don't do stupid crap like that. n/o to its hosts but that was the weirdest design to have half the game cut off from the other, the WCs didn't overlap at all.
Unfortunately I've found that the faction leader system is the same as the village leader system in that, if the team is ununited, until they are united, you are probably just told orders and left in the dark unless, like the village, you have a relevant/probably-not-missing role like hooker or BG, or you are a mole yourself for your team and they assume you must be legit. So until the team is united, faction leader doesn't particularly differentiate itself from village leader as you still have the untrusted outskirts of the faction kept in the dark from the vital-role inside circle, of course, this is minimized when the team is just plain too small to either have superfluous roles or really keep anyone in the dark.
Anyway ununited factions kind of work out really poorly in actual play. In MP2, the idiout Light Aether sat around with a mole and a recruit and they couldn't figure either out, or even clue in they HAD a mole, for eons, until they were wiped out. Everyone else was p alright but the LA were really complete tools.
Basically inactivity in ununited allows the moles, who are usually active people b/c of the role description, to sweep through a faction, and the faction leader, well, in one game you could VOTE for the mole as the leader and that did happen, p sure I went off on a rant at that game about how "voting for leaders when the faction is ununited is the worst ever omg you are all horrible people diaf". In that instance, the mole did gain the entire faction's trust enough to get the team to vote them for leader, or at least, told the "faction leader" actual member to vote them for whatever reason and because of the leader system, the outskirts faction members obeyed orders and bam mole.
tl;dr if you want to do multifaction, unite the factions from the start ~_~ you will be a lot less pissed, and instead of just "kill w/e" give the factions some interesting/tricky WCs to achieve, so while united, they still have a bit of an uphill struggle (like the temples in MP2 or finding the non-troll, actually-useful ALLCAPS). Faction leader is just as easily moled from the games I've played and is: still boring for those of you who don't have convincing-enough roles until inspected.
Unless you have strong failsafes to prevent the mole from becoming the faction leader as voted upon, it's just going to result in a lot of the same until the mole is found out regardless of how the uniting system works. I admit RotS' was a lot better than EXPERTS' in that you could partially unite and at least get, you know, SOME of the team assembled, but even then they came in sets of 2/2/1 so it was a lot easier to determine who is molesauce.
@Daenym the letter idea is good, but, once again - if someone is inactive, never claims, etc. or does just ENOUGH to send in actions and BE active you can't fairly sub them, it will delay their faction sooo badly and severely mess them up. Even expert games suffer from inactivity (in fact, EXPERTS has the worst activity record of any of my games), so while using letters or something similar to unite is a good concept (ours are all linked by PRs which lets the moles in as well), if someone doesn't step up to the plate, their faction just unfairly fell behind, and unless people ignore them (in favor of killing the active people off, which was the problem in MUDS as anyone who seemed to lynch and be in the channel was killed due to 'big threat') they will likely never catch up.
Actually p sure the Really Bad Artists had this as "story of our life" and then nobody cared so they lived to the end.
ok I'm done but yeah ununited is problematic having played and hosted plz avoid this just be INVENTIVE with your WCs so they can start united and have fun stuff to do while they kill enemies!!