Pugna is my recently most played hero and Aghs just isn't that great on him. It only lightly increases the damage and the zero cooldown is entirely situational. If you're doing extremely well already and you're up against the right lineup, it can be damn good. But in a tough game you're rarely going to suck people off 3-4 in a row. If you're getting interrupted to begin with, that in itself might be the bigger issue or you just realize casting your other spells, particularly decrepify, is more important.
Aether Lens, Octarine Core, Lotus Orb, Rod of Atos, Euls, Mek, Glimmer Cape, Hex and so on usually provide you with more options. There's pretty much never a game where I think I should get Aghs above any of these others.
The utility Pugna provides when he has Decrepify, Lotus Orb, Rod of Atos and Glimmer Cape is insane. Add Aether Lens and Ocatrine for range and CDs and it's nigh impossible to deal with.
edit:
Btw, it's weird how I can't select the top path anymore at all. Probably weirdly coded since there's no quest left to select and how that would mess up. Also I got the question for revealing and killing 10 invis heroes after getting just one. Can't say I can complain about that.
Maybe its because I never play her but that one seems a bit tough.
The cooldown is actually so low that over the course of a decently long game it's not that hard to achieve actually, especially if you manage to get some 2-3 man stuns. I just practiced the Euls timing in demo mode so that meant I pretty much never failed that combo either.
Still so, was kinda lucky. The match must have had an average MMR quite below mines so even though I did rather badly on Lina, though in my defense I had to go offlane and was up against heroes like Tusk and AM, we managed to win eventually.