"worked fine when the battery voltage was up, but it sucked when the voltage was getting low. " I would suspect that was the sign of a cheap "bulb". An LED can provide uniform lighting over a wide voltage range, but only if there is a regulator chip in the package. That chip may cost as much as a cheap LED itself does, doubling the net cost, so you won't see it in cheaper designs. They'll use a plain resistor to limit power instead.
There's a similar price to be paid for consistent "color" in white LEDs. Those Nichia-patented white LEDs have some manufacturing problems. The process produces LEDs that are all "the same" but they fall into nine very different end results. The color varies cool-neutral-warm into three groups, and the buyer has to pay extra to get LEDs sorted by group, instead of the whole variation. The brightness also varies, roughly 1x-2x-4x and if you want all of them to be the same brightness? That's right, you pay again.
Nichia will sell "assorted" as they come off the line, or hand-tested in any of the 9 (3x3) quality grades. Other vendors may have more uniform production lines, but every LED product has its own unique quirks, either color, brightness,power...something.
Of course that's not unique to LEDs. Even with tungsten bulbs, there are "long life" "industrial" "burn base up/down" "burn sideways" and the ever so special cheap imported ones that last less than six months. We're just used to tungsten bulbs being cheap consumables, easily replaced. (Ain't so cheap anymore, either.)