Sometimes super-specific domain knowledge shows up in surprising places. Tubular Li-Ion cells (e.g. 18650s, 20700, 21700) are widely used in a LOT of stuff, from power tool battery packs, to electric cars, flashlights, and specifically: electronic cigarettes.
Electronic cigarettes are an interesting application, because they require a LOT of current, typically 10-75W or so, which can be as much as 20 Amps of current in short bursts. They also need to have a relatively high capacity (so you aren't charging it all of the time), as well as be relatively safe (it's near your face).
Before e-cigs, the only application you'd typically buy 18650 cells as a consumer was for high powered flashlights (fun fact, a lot of e-cig history actually derives from high power flashlights, the reason e-cigs are typically called "mods", is that they were originally "modifications" of high power flashlight bodies).
As flashlights were a relatively small and niche market, there weren't a lot of objective "ratings" for cells, which could generally be WAY overstated, especially from shady (read: eBay and AliExpress) vendors. When e-cigs started using these overrated batteries, it could range from disappointing to dangerous.
One of the coolest resources nowadays is from a guy named Mooch315, who is an electrical engineer who does lab-style testing on batteries. He posts his findings, and is pretty much the ONLY source I trust when choosing Lithium Ion batteries, for e-cig purposes or other electronics usages. He even tracks how popular cells are cloned (with substandard replacements), and how to spot fakes.
Here's a link to his forum post, which he regularly updates with recommendations:
