I can only comment on certain themes so far.
Animal - This theme is actually pretty good. It's basic. Nothing special but nothing bad or broken. SC4 summon seems a little underwhelming but not horribly so.
Rat - This theme is pretty weak at SC1 and 2. SC3 and 4 summons are cool and thematic.
Ooze - This theme is fine. I'm not really sure how -good- some of the summons are, but they certainly add diversity and are thematic. Low AC hinders some of the summons mechanically but they're ooze so it's fine.
Vermin - This theme is by far the strongest of the four that I've used so far. Comparing the SC2 from this with the SC2 from rat, you see 20+ more hp, much higher AC, higher AB, much higher damage. The SC3 from this theme is very powerful but balanced because it's a bit buggy (no pun intended). The SC4 from this theme can solo quest bosses on 6-9 quests with just a couple of buffs.
That being said from what I've heard Vermin is not even among the best themes.
Summons are definitely very powerful at the moment. SC4 creates summons that rival if not outright beat geared level 7 fighters in raw stats. I'm really enjoying playing a summoner but that might need to be looked at a bit.
Edit: I did realize that summons get buffs when you are fully built as a summoner. I'm talking about summons post-buffs. They might be less powerful when used by regular, not GSF Conjuration characters.