Summoning only scales off the summoning ability and your level. Early on this is really overpowered. A (lone wolf) full summoner can beat Dallis and her companions at the Fort Joy gate and later on the ship in Act 1 without any problem. But when you hit 10 in summoning ability you have no other way to improve your summons and they start to lag behind. It's still really safe and does its job, but the damage is lacking.
Doing damage yourself, which scales off multiple abilities, gear and attributes is so much stronger. I respecced at around level 12. At this point I had 26 in summoning ability on my lone wolf characters (10 base + 10 from lone wolf + 6 from gear) and I buffed my incarnate / bone widow with all available infusions and buff spells from all spell schools.
I respecced then my characters to a twohanded knight and a ranger. Sure they felt initially squishier, but compared to the fully buffed summons on my full summoner, they also did double damage per attack and have more AP to attack. And the damage gap between these playstyles gets larger and larger with every levelup.
