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.
You just reminded me Summoners don't have many spell slots. So they're limited as martial characters, but more so as magical characters because they can't cast tenth level spells, and then on top of that they only have four slots for regular spells at most.
Yeah. In my experience (as a GM with a summoner player, going through Strength of Thousands, currently at level 12), Summoners spend the majority of their time in combat acting as a 'fighter', with the summoner spending their 'at least 1 of the 4 actions' casting Boost Eidolon. Which, on that front, makes them a 'worse fighter'.
But, of course, they can throw out big spells when needed, since while they only have 4 spell slots, they don't lag behind a full caster at all in 'highest level spellslot' (aside from 10th level spells). They get fireball at the same time the Wizard does. So the real 'breaking point' between them an a full caster isn't 'burst power', but being able to lay out a constant barrage of lower level spells, meaning they lose out a lot on utility.
But, staffs on them are very important, as that is huge in giving them those 'lower level utility spells'.
And, unlike a fighter, they do have access to damage cantrips, for both when ranged combat is needed, and if elemental damage is needed. Sure, those damage cantrips are worse DPS in a white-room scenario most of the time, but they are nice to have when needed.
So while they are spending 90% of their time in combat mostly being a 'worse fighter', 10% of the time they're throwing out spells as strong as a Wizard, and just generally bringing less DPS but more flexibility than a fighter would. In that regard, they're maybe more analogous to an Inventor.
Which, on the proficiency front, the Eidolon shares martial proficiency progression with most martials, so they'll lag behind fighter and gunslinger on attacks, and monks/champions on defense, but keep pace with rangers and rogues and swashbucklers etc.
Summoners as a caster though lag behind full casters a bit, getting expert and master spellcasting 2 level later (so, more levels than not, they'll actually have the same proficiency as a full caster). More painful though is they never get Legendary casting, since full casters get that at level 19. While I've not yet seen summoners at that level, having your DC/spell attack be 2 lower is painful, but able to be mitigated by focusing more on 'party buffs'.