The shaman also feels underpowered, on several levels. The basic abilities aren't very good, the spell list is bad, and the special abilities that define the class are underwhelming.
The basic chassis
Shamans are among the physically weakest templates, with ST10. They're also the lowest DX template, at DX11. They're the least smart of the "smart" templates, with IQ13 (even the maligned bard is smarter!). The shaman is clearly not a melee combat or a stealthy backstabber; a shaman is going to need to be protected in combat. They even have worse melee combat skills than a wizard.
Despite being described as expert negotiators and diplomats, shamans don't have Charisma as a required advantage. Diplomacy isn't a primary or secondary skill: it's an optional background skill.
Shamans don't get Herbal Lore or Esoteric Medicine, but they do get First Aid and Pharmacy (Herbal). They can bandage wounds and prepare antitoxins for poisons, but not provide general healing or healing potions.
They do get Naturalist and Hidden Lore (Spirits). Naturalist is a good monster identification skill and has some other uses in outdoor adventures. Hidden Lore (Spirits) is probably the most obscure Hidden Lore skill, but it is the kind of skill you'd expect from shamans. The lack of other Hidden Lore skills means that the shaman can't substitute for a wizard very well.
Shamans are, in theory, spell-casters. With IQ 13 and Power Investiture 2, they're not very good spell-casters, which is only acerbated by having a mere 8 points in spells. Their spell list is composed of obscure utility spells such as Astral Vision and Sense Spirit. At PI6, they get moderately useful spells like Create Elemental and Beast Summoning, spells that other casters got with much less investment. Despite being described as possible replacement for clerics, the only healing spell they get is Cure Disease, and even that requires PI4.
Shaman spells are unaffected by mana or sanctity, but are "reduced in effectiveness in places that are somehow barred to spirits." This restriction is both flavorless and maddeningly vague for the GM.
All shamans get Channeling, Medium, and Spirit Empathy: 30 points of advantages that pretty much translate to "you can see, talk to, and try to influence spirits." Completely appropriate. They also get 90 more points of discretionary advantages, which can either be used to buy various allies, shamanic gifts, or mundane advantages like Charisma. The mundane advantages aren't very impressive, and include such depressing oddities as ST or HT +1 (but not IQ or DX!) and Power Investiture 3. Going back to spells, it's sad that the default Shaman isn't even allowed to purchase enough Power Investiture to get the better spells.
Shamans have a power modifier which is basically a Pact. Fair enough. The actual abilities aren't very impressive: Active Luck, See Invisible (Spirits), Speak with Animals, True Faithing (without Turning, so a Shaman can protect himself but no one else). Dark Vision is probably the winner here, but given the shaman's generally weak abilities, sending one off by himself to scout is a cruel joke.
Shamans can take a range of allies, from indentured petty demons that are almost free (2 points each!) to potent elemental allies (15- less, summonable, for 30 points each). Shamans are summoners, so it makes sense that allies would be their reason for being.
In play, shamanic allies aren't bad, but they're not great. A lot of them are Diffuse, which can be helpful when fighting a lot of typical DF enemies (most orcs do not have area effect attacks). Still, most of the sample allies in DF: Summoners are weak in play: low Perception, low Will, low DX, and low attack skills can make for an ally that gets surprised in ambush situations and can't hit anything when it finally attacks.
Even with hand-crafted allies with more power, the shaman isn't that much more impressive. A demonologist can have demon allies, an elementalist can have elemental allies, and a necromancer can have undead allies. The shaman can have any of those allies, and so if there's some special synergy in having a ghost, a demon, and a fire elemental, the shaman brings something unique to the table. If there's no synergy, it's just as easy to replace the shaman with one of the other summoners (or a druid, cleric, or wizard) with the appropriate ally.
Shamans are a bit disappointing. What could have been a mix of Bard, Cleric, and Wizard - basically, a replacement healer and face-man with some utility magic, excellent for small delving bands - is instead a highly focused expert on spirits whose main combat utility is whatever his Allies can do. In an adventure with a lot of focus on spirits, a shaman is really powerful, but as a general delver, the shaman doesn't bring much to the table.
In a 5 person delving band, running through a conversion of a standard AD&D module, the successful band is going to be some collection of (Knight, Barbarian, Mystic Knight, Swashbuckler, or Holy Warrior), (Cleric or Druid), (Scout or Bard), (Thief or Artificer), and (Wizard, Sage, Necromancer, Elementalist, or Demonologist). The shaman can't easily replace any of those roles. He's a specialist, brought in to deal with spirits, but not a regular member of the delving band.