Going by the lumen output on the datasheet only provides an upper limit to the light output. It's where you get $30 lights that claim to be 1600 lumens. The LED can theoretically put out that much, if you have perfect optics and a perfect driver and have a huge heat sink that can keep the LED at the perfect operating temperature. Just something to keep in mind.
All that said, I think probably some of these cheaper lights do provide good light output. What I think they lack is good design and durability. They are unlikely to have excellent mounting systems, and they are unlikely to work for many years.
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Work: the 8 hours that separates bike rides.