So, at a high level of play, the Summoner Wars app (with 10 factions, as distinguished from the physical game with more summoners) has a very discernable meta.
7 of the factions have catch-up events. 5 of them are the dreaded Magic Drain. (Of the 14 unreleased summoners, only 2 more have catch up events and neither is Magic Drain, so it's a big difference.)
Catch-up events, especially the devastating Magic Drain, encourage self-kills, low unit counts, and waiting for the other guy to do something. The non Magic Drain matchups are as much of a blast as anything in the game. And I don't mind the Tundra Orcs' or Sera's CUEs at all. (Reinforcements and Summoning Surge.) Heck, even the fact that Ret and Vlox have Magic Drain is countered by the fact that they're not top tier, and even when they dominate, it's usually due to aggression, not passivity.
However, Oldin/Elien/Tacullu cause problems, as they have the most un-fun and game-warping CUE (Magic Drain) and also are VERY good decks when they sit back and turtle, sometimes with self-kills. So there is a lot of motivation to play them, and to play them in (subjectively) less fun ways.
Here's how I expect the unreleased 6 first summoners to affect the game:
Only two more catch-up events (another Summoning Surge and an event that discards cheap commons from the board), and neither of them is Magic Drain. So a higher percentage of the matchups will just be more fun.
Sand Goblins, Filth, and Deep Dwarves are likely to be right up there with the "powerful" factions currently. Swamp Orcs have a chance too, but the Swamp Orcs' biggest problem is Guild Dwarves and also Rune Mages (see below). Over all the balance of these factions is even better than what's out there now.
Filth can be played with extreme deckbuilds (heavy emphasis on big champs and magic generation, or no champs). We will see if any of those wacky builds are easy enough for people to comprehend and pull off that they become a dominant strategy. Part of the self-kill/turtling Oldin/Elien/Tacullu problem is that those strategies are strong, easy, and don't take much deckbuilding know-how.
Finally, the notorious RUNE MAGE. This is a common merc that can forgo its movement if adjacent to an opponent wall in order to steal one magic from the opponent's magic pile. People are afraid these things will be in every deck, thus causing non-fun games when people get one locked in that can't be easily destroyed. Here's my thoughts on Rune Mages on the app:
Hopefully they are implemented with what is the official ruling, that due to the "moving WITH this Rune Mage" wording, they can only siphon in place of of their main movement activation (ie not via Controllers, Kynder, etc). If they are implemented this way, this gets rid of some of the worst negative play experiences right there.
I do think they will be very common in competitive play; however, there are ways to deal with them for most factions.
I do not thing their effect will be all bad. Although they will discourage people from placing front-line walls (that is bad), they also force you to come up and play a mid-board game, so, for example, they can help bring Elien out of his hole. We'll see.
Even though I have played plenty of games with all of these summoners, some of what I'm saying is still speculation because once a faction shows up on the app you can get tons more plays in with them and have a more solid feel for them than in real life.