It's engines are old, at least 1.5 generations behind the latest engines in production. Fuel efficiency only looks low when compared to aircraft using more modern engines. Unsurprisingly, Emirates canceled their order only after Rolls-Royce rescinded their plans for an engine upgrade. That's why there was the one year of waffling; Airbus and Emirates were trying secure a commitment from Rolls-Royce as everything turned on an upgraded engine.

Even with the engine handicap the A380 was still generally competitive if fully loaded. And if not fully loaded it could still be competitive, as described by a recent story about the outsized role of the Dallas-Sydney route to Quantas' bottom line: https://simpleflying.com/dallas-sydney-qantas/ (see also https://www.dallasnews.com/business/airlines/2019/10/28/this...)

Considering that engine efficiency is perhaps the biggest contributor to financial viability, that says something very positive about the A380 approach and design. But as engines continued to advance the A380 would have lost even that edge.

Yeah, the A380 designed for a longer fuselage so it's inefficient in the -800 variant.

Makes sense. It looks too bulky, and not slender enough.

Can you expand on that?