My opinion is that the conjuration sphere is balanced overall. Yes, it's a bit nerfed, but I think it's made up for with the fact that you can heavily customize a companion or even give it access to spheres as well as monster feats.
Take this build, for example. A level 1 Soul Weaver (I helped design that class, BTW) chooses to begin with the Deathsphere, and the drawback to never you Ghost Strike. (so can only animate dead) giving her 3 talents. She chooses the conjuration sphere, which comes with one form talent for her first companion. She decides to make a Wolf called "Fury". She uses it's bonus form as "Altered Size" to make it large and uses her remaining two talents to give it Elemental Creature (fire) and Magical Companion.
Fury, being a level 1 caster, has one feat which he decides to use on Spheres. His first Sphere is weather, but takes the drawback to be restricted to only increasing heat. He now has his talent back. He then takes the Destruction sphere and takes the drawback to be restricted to close range attacks only and the drawback to using only one one element for Destructive Blast, which he sets to be fire. He then takes "Energy Blade" with his remaining feat.
The Soul Weaver, as a human, gets two feats to start with. She decides to put both of them into "Extra Magical Talent" which allows her to have her companion out all the time.
This min maxing is very powerful, yes, but ultimately you end up with a level 1 character who put all of her abilities into the creation of a single companion. She only has to use 3 spell points a day for a companion that can can deal 2D6 + 1D3 + 3 bite damage at a time, which is VERY powerful, especially since the target can also catch fire, but that's pretty much the only thing she can do. It is more expensive than just getting an animal companion, but it has the advantage of more damage and better control so I think that it's a better choice than an animal companion. It can also talk!
From there though it's power will mellow off. The companion gains magical levels slowly so it would probably be better off just going for feats. For example, at caster level 3 it will get a new feat which it could use for "Jaw Lock" to allow it to grapple an opponent it bites thus giving it a chance to do more than just damage, but this won't deal any more damage either. Unless the caster wants to put more and more talents into that companion it will eventually be reduced to a mount with limited battle function - although definitely a good place for a summoner to be!
I disagree with the idea that the conjurer can create an unstoppable army though. The form abilities have to be taken for every single new companion.
So, for example, let's say that at level 2 she decides she wants a new companion. It will come with a form ability of it's own, so lets say she uses that form to give it Lingering Companion. Combined with the Greater Summoning ability which does apply to all Companions she really could have a new companion every single level. By level 20 she would have 29 companions each with 15 HD! Or if she uses all her feats to get new companions she can even have a total of 25! However, they would be little more than meat shields. With standard stats and starting abilities, each would have 1D6 damage per attack. If she put all ability score adjustments into strength it would be a mere 1D6 + 3 damage tops per companion *AND* she would have to spend 60 spell points to actually be able to summon them all! The problem is she will never get that many! Even if she rolled an 18 on her charisma, and put every modifier into it, she would never have enough spell points to Summon all of her companions. Even if she could, and even if she had one riding another (think Minions riding wolves in Overlord - which would be really cool, come to think of it!) there would be too many to fit on a typical battlefield.
But lets say that she gets to level 20, puts a point in charisma every time she gets an ability score increase, and wears a Cloak of Epic Charisma (+9) at level 20. Her charisma score would be 32 which would make her modifier +11 which would allow her 31 spell points a day. Half the points she would need.
If she instead took 9 feats in "Extra Skill Points" she could have at most 49 spell points at level 20 (including maxed ability scores and cloak of charisma +9) which would allow her to summon a maximum of 16 companions (17 if she used her two bonus feats for more spell points), but she herself would be vulnerable, would not be able to do anything herself, and although there are class abilities that could be particularly damaging (10d6 channel negative energy) when you compare this to what another character can do at the same level (10D6 Destructive blast that is immune to antimagic fields and ignores spell resistance) it would not be in her best interest to conjure an army. (in fact, said destructive blast set as a burst could destroy that entire army and you with it all in one attack!) She would certainly benefit from having multiple companions, and it would be kind of cool to see one companion riding another, but the most I would make would be, maybe, 7 companions. One to ride myself, and 3 to ride the remaining 3. It would only cost 21 spell points to call that many and if I invest in a Death Sphere ability every other level and/or augment the power of the companions I could with that combination have an army of sorts and it would be more balanced with another character of the same level.
Ultimately, this example shows that even with min/maxing to get the most number of companions with the most power it is still balanced in the context of the game.
Although over powered could mean many things. For example, I designed a "Wonderous Ring of Tavern Summoning" which can call into being a finely crafted Tavern for 16 hours a day, where all the materials come from one portable hole, and to have a store room with up to 120 pounds of food and mead which can be restocked, all for the low price of 78,000 Gold. Seriously, the ability to summon a tavern from a ring to always have a place to stay would be amazing and would probably pay for itself.
