Jesus' beef with the zombie Christians of Sardis is not founded upon mere conjecture. He's seen them in action, he's witnessed what they have done. They had a name, or reputation, for being a vibrant community of Christians, alive in God, but their actions did not measure up to their hype. It's not that they did nothing, they at least made a feint in following through with the works of God, but they stopped short and took a nap before they had pressed through to true fruitfulness (completeness) in God.
What the actual nature of their shortcomings was Jesus did not say. Conjecture and personal application is left to us; in my case, it reminds me of the Parable of the Sower. Rocky soil or weedy soil has the start of something good in reaction to the gospel, an appearance of life, but whatever the initially promising impact may have been, it is not followed through on with dedication and focus. What began to grow withers or remains fruitless (incomplete) as a result.
I look at this and wonder if Jesus could have been writing to the Protestant Church. The promise of a return to the Word and the reformation of what had become mere superstition, idolatry and commercialism certainly gave the name of life to the Protestant Church. However, given her history and current state of affairs, what would Jesus say of her works? Protestants have fallen asleep in the light while whatever spiritual life and light they may have had has ebbed away. The promise of greatness for that body is all but past, now it would be sufficient if she just woke up, came to her senses, and bolstered what remained.