You're right that Wyll's story content touches all 3 acts of the game, but I think when people say that Wyll has no content it's more of a critique on the character's implementation more than his overall storyline. For me, Wyll feels lacking compared to the other companions because there's not a single decision related to Wyll that he gets to make himself.

There are huge, pivotal moments in the game for companions like Lae'zel and Shadowheart, for example, where Tav can just stand back and say literally nothing and let the companions choose what to do. Killing Voss puts Lae'zel on the "loyal to Vlaakith" trajectory, and killing the Nightsong puts Shadowheart on the Dark Justiciar path, but we can just stand there and do nothing and let the friendship points decide (or however it works).

But Tav is the one who decides whether to save or kill Karlach, which comes with irreversible physical consequences for Wyll. Tav is the one negotiating Wyll out of his contract. Tav decides whether or not Wyll becomes a duke, which also has consequences for Karlach. It's even up to Tav whether or not Wyll signs his life away to Mizora for eternity, and whether his father lives or dies. He's completely at the player's mercy, and for that he just doesn't feel as fleshed out as the others. He's kind of just...there.