The main advantage of monoclan decks is their stability and support: cards. You're assured of getting a bonus on every card, none of those unlucky 3-1 splits. For clans with powerful support: cards (emeth, Buba, Oscar), this adds further incentive to go mono. Many clans like Rescue, La Junta and Sentinel work very well as monos.
The drawback is, support: notwithstanding, you're using inferior cards to fill out your deck. There are only so many usable cards in every clan, trying to pick 8 cards will likely result in the usage of subpar cards as fillers. Andsome, Corrina or Jerry are seldom used in half decks, but for monos, you have to consider them, especially when elo bans a handful of staples. In fact, it's very hard to make a mono this week due to the bannings.
Another drawback is the rigidity of monos. Many clans have glaring flaws: Sakrohm is vulnerable to SOAs, Nightmare lacks good DRs, Freaks bonus only works 1 turn. Matching two half-decks that cover each others' weakness or bolsters each others' strength can result in more balanced decks. Of course, there are clans that are already pretty balanced in mono....Rescue and Piranas for example.
As for leaders, most of the good ones are unfortunately permabanned in elo. It is still possible to make good decks with the usable leaders, such as Bridget-Pussycats or Ashigaru-Vortex, but they are a bit gimmicky and rare.
The problem with mono decks is all their big guns with Support abilities get destroyed by SOA (Skeelz being the exception but Skeelz's 5* characters are disasters in ELO). If you take something like Jungo, that clan has a split personality- it has Support but it also has Stop characters- so which do you run? So that is a huge problem.
I like 5-3 split decks. 5 characters gives you plenty of stability without relying on Support: characters. 3 is fine for clans where the bonus doesn't matter all the time (Pussycats, Vortex, Skeelz, Junkz, Sakrohm) or that you don't want to draw 3 characters ever (Freaks). a 3 character Pussycat splash of Yayoi, Britany and Cassio (8*), Skeelz of Chiara, Redra and Liam (7*), or Vortex of C Wing, Neloe and Onyx (7*) brings a lot of power at bargain efficiency. While the Vortex bonus is nice enough that you wont be hurt for going to 4 characters, adding a 5th character to a clan that has a great bonus more than makes up for it. For instance I would never play Uppers with less than 5 characters.