I see the Seleucids as an interesting mid - and even late - game challenge. Yes, it is tricky to defeat them when playing Pontus or similar, but it is doable. (I played a recent game as Pontus, and did well.) Are they fairly balanced? Maybe not, but I do not see it as huge problem. As for an expansion, all I can say is that I personally would love to do another one.

There is a lot of mileage left in EU:Rome.
I recently had to end a game with Sparta because I ended up being steamrolled after having to concede a defeat, despite having laid claim to the entirety of Anatolia. The fact remains that, in the early game, the Seleucid Empire is very difficult to bring down -- the patch I'm playing 2.31b seems to have the Seleucids bringing the Egyptian Ptolemies and the Armenians into tributary-allied status, with all three able to field rather large armies -- and from then on can effectively defeat any other power within its sphere of influence (Black Sea region, as I've seen), with the advantage of large stacks and huge amounts of territory. The issue is that, when a huge region of territory is brought under foreign control (in my case, Anatolia), even if you are able to conquer another huge swath, you'll be limited to getting only a few provinces. Even if it's a failure on my part to deal with the Seleucids, it still feels as though playing any power in the Black Sea region, or with expansion options that are forced towards the Black Sea region, the Seleucids end up being such a problem that those powers just aren't worth playing.
I would imagine that, historically, even a large empire like the Seleucid Empire would have to concede a defeat if enemy forces control such a large amount of their territory. Having eight stacks of forty-thousand, all with above 7 generals, and no territory lost on your side, yet still having to give tribute to the big yellow blob, just seems as though the Seleucids have a major advantage, preventing any successful campaigns against them.
Also, even with the Parthian League appearing relatively early, in that game, the Seleucids were not weakened in the slightest. They simply eliminated the league without losing anything.
To sum up, I guess I would have to say that I feel as though something should be done so that the Seleucids can't just lose a large section of their territory in a war, and still be able to force a concession on the other party's side. And after that war, there's nothing much that can be done on your part, as they will continually declare war on you afterwards. To me, having no room to maneuver in the Black Sea region is just a dealbreaker, and keeps me from gleaning any enjoyment from the Greek states, Colchis, Armenia, Rhoxolani, Bosporus, Pontus, Bithynia, or Pergammon.