Warpriest since the changes to divine power/divine favor seems like it does fine solo against a lot of content in Skal at least. I've been able to do the myconids at level 10 and minotaurs at 13 alone due to riding a constant high of divine favor and divine power -- the constantly refreshing temp HP of divine power keeps Warpriest afloat. Based on my previous experiences on Skal, I don't think I'm going to struggle with anything solo other than perhaps the dragon -- if only because it casts Disjunction, which is basically the anti-warpriest.
The particular build I'm going with gimps itself until about level 26, too, as the weapon I'm currently using is 1d6 and x2 crit until I manage to get proficiency in the weapons I really want. Despite this, I feel fairly capable of preparing for any challenge thrown at me.
Because of this, at least for leveling from 1-20, I don't think warpriest needs summons as long as the player of the warpriest knows when to heal, what consumables to bring, and how to ration out their spells properly. I'll probably post something else when I start getting to the epic content, because I think there, the story probably starts to be different, especially with places like the Reavers and the Shadowplane writs, which can be vicious alone.
Seeker is in a weirder, more awkward spot. I think it might be able to suffice off of crossclassing AE with the new skillpoints it gets in order to deal with the difficult early level curve, but mid-level and late level are still going to be incredibly tough. It could probably still use limited access to summons.
Edit:
One caveat is that warpriest suffers from the problem that most gishes suffer from, but since you don't have a summon, it tends to be exacerbated. Enemies that can KD you -will- KD you. It doesn't matter if their AB is 20 or 6. If they crit, you're going down. This can be pretty damn dangerous in larger packs of enemies, and you have no way to mitigate this until you take your dip. This is incredibly dangerous solo, and can often lead to death. I've been lucky enough that most of the knockdowns I've experienced have been harmless, but there is the constant threat of getting KD'd in the wrong pack of enemies.
I don't really know if there's a good solution, but I do think that it sucks that you're so vulnerable until you take a discipline dip. Seeker suffers in the same way, of course, but the seeker playstyle, at least from my experience, lends itself better to cheekier tactics where dealing with an entire group of enemies is just never a problem.
Edit 2 -- The Revenging:
This also means that if you ever see a pre-epic warpriest and want to kill them, the first thing you should do is KD-check them. If you manage to beat their AC, you win the fight.
