Of all the films he's made, the only two I really give a shit about are Desperado and From Dusk Til Dawn. Once Upon a Time in Mexico is also pretty good but it's not really on the level of what came before it.
Like I said, I think Sin City is a technically well-made film but on a narrative level it bored me. There's also something about the way the story is told that just lacks energy. I was hyped for it when it first came out because I thought it looked awesome from the trailers, but I was basically busting one of these in the theater:
Well, I think you're right that the concept can only be taken so far. But I think that choosing the right stories to tell is part of the responsibility of being a director. He could've bypassed the Machete concept in favor of telling a different story but he didn't.
I will give the first Machete credit for all the scenes with Steven Seagal though. That shit was hilarious.