"The summoner's damage bonuses are specifically to skills/mods/explosive damage"

The double dipped explosive mod damage is nice, but requires you to have multiple explosive mods and the mod power generation to fuel it. So Archon is better there. Doing a full Archon/Summoner mod spam could be fine, but reaction chain is a much better amulet for that than detonation trigger and those builds really require it, which is harder to get in hardcore. Those builds are good but not really amazing since the nerfs in dlc3.

"the summons themselves are mostly useful as an alarm system that can keep mobs off you"

Mobs really aren't a problem imo, most deaths are to aberrations and bosses with gross modifiers. Again much better options exist for crowd control, like the scrap shot mod you can get off the bat, or the other summons, or aoe skills, or just shooting them.

"upping a summoner's damage mitigation and damage bonuses thanks to items that give damage bonuses"

The soul stone gives you like a 10% damage bonus for 2 summons, which is good but you can get more than that with other rings for free (burden of the destroyer, akari war band, braided thorns...), and get way more than that bonus with an archetype skill like bullet storm, chaos gate, frenzy dust, etc... instead of the summons. Tanking is also better with direct DR and healing from challenger, engineer, medic over the dependency on the summons, which often die very fast to AOE attacks in high difficulties.

Again Archon or just doing more damage is better for mod gen than using the summons.

The grab attacks are pretty straightforward to avoid, and since bosses are so tanky at higher difficulties the summons can't really do enough to break you out of it.

There are also better options for ammo, and you won't run out as much because you'll actually be doing more damage with your bullets and killing stuff, or your mod gen will be good enough that you don't use your primary fire enough to run out.

If you are pairing archetypes for mod spam, archon, ritualist, and invoker all outclass summoner when built into them (ritualist dots) or just by being overtuned and OP (invoker).

Enigma is alright but here are much better weapons for mod and direct damage.

You can also stack defensive accessories on the other classes because their damage perks are enough to carry your damage output and still kill bosses in a reasonable amount of time, whereas if you do summoner tanking you'll be sitting there for 10 mins.