Yes, I think Agonizer is very mediocre and win-more for a signature attachment-- I suspect the "you used to be able to put it on your Warlord but it got nerfed late in development" proposal rings true. Consider that Ion Rifle costs the same amount and consistently gives +3 attack. The only times when Agonizer is going to be much better than that is when you have a ton of Khymerae that your opponent hasn't been able to counter-- in which case you're probably winning anyway.
That said, no card with 3 shield icons is bad at this stage in the game, and I've won games on the back of an Agonizer + Hypex Injector Mandrake-- but I do think that most of the other signature attachments are better than Agonizer. In general I see it played as a shield more often than I see it actually deployed.