A YouTube video I watched recently claimed that summoners might have been considered a bad class in second edition. However, it doesn't seem like that at a glance - there's opportunities for various creatures and beings to stand by your side, such as a plant monster kinda like Blossomon, a stegosaurus, dragons, and so on. Also they come with a nine level magic collection on top of the Eidolon - and the Eidolon can learn spells too with the correct feats. So you could make a druid like character with lots of magic but their main aspect being able to summon a giant sunflower, as a result of acquiring lots of Druid archetype feats for magic casting and feats related to the Eidolon casting magic themselves. Or you could make a stegosaurus man who summons a powerful stegosaurus that knows a lot about nature and fights for them. Sounds like a cool class to play, but I haven't tried the game much to see how any of this would work in gameplay.
I meant more in 'opportunity'. Like, casters usually suck when they're in situations where they're just throwing out cantrips, and melee fighters suck at times when ranged combat is needed, or when utility/AOE/Elemental spells are needed.
So they can leverage their flexibility to throw 3 actions at the 'most relevant choice' between a 'fighter' and a 'wizard'. Granted, either form is going to be weaker than a PC Fighter or Wizard.
Add to that their occasional feats like 'Tandem Movement' that lets them kinda sorta cheese the action economy, and things like 'Eidolon's Opportunity', letting the Eidolon threaten spaces even while the summoner is acting more in 'Wizard mode', and I think an average effectiveness of 2/3s is fair. Yeah, they do lose out on 2 of 6 actions compared to two full characters, but that just means they dump the 2 'least useful actions' rather than straightforwardly being 2/6th worse.