Interesting.
I think every one of the 'lesser' factions has it's place in todays game. In fact, they're all probably just 1-2 pieces away from truly matching it with the big boys.
The Mandos are excellent against the rebels/NR because they kind of don't care about activation control. They'll get you with gunslingers (cunning) or scouts (opportunist) either way. The R7 and mouse droids have really helped them. Of course, the Lancer and Yobuck are the achillies heel for mandos. But given how popular Dodonna squads are, they arn't a bad choice, but you need to build them right and play them well.
The Sith are the closest IMO. If Dodonna didn't exist, they'd be tier 1 already I think. He is their major weakness. Offensivly, they are great, and pieces like Dash and Zuckuss have really boosted them. Probably the best addition to that faction was the non-unique figs they got from JA. The Sith Lord and Apprentice are fantastic support pieces.
The Vong are interesting. They actually counter things like Yobuck and the Lancer really well with Nom BOmbs. Cad Bane is a great fit for the faction. Again, like the mandos, the R7 and the mouse droid have really helped the Vong. Unfortunatly, outside of Nom Bombs, its hard to make a competitive Vong squad, but they do have excellent anti-melee stuff, and given how powerful melee has become, that counts for a lot I think.
The OR is always seen as the worst faction, but I'm not sure they are. There is actually a lot of power there. The Exile and Lord Hoth are great commanders, extreamely cheap for what they give their faction. Atton Rand is also a great fig. The OR has some excellent non-unique pieces too, the Jedi Battlemaster for example. A lot of the more recent fringe options work very well with OR too, like Cad Bane, Zuckuss, Dash and the TBSV. With a little though, you can build OR squads that have scary damage potential, they're closer than people realise.
Hopefully, the V-set will build on the what these factions have, and pretty soon, we'll have 9 fully competitive factions to choose from.