The LEDs don't just not last as long as advertised, they don't even last as long for me as the approximate incandescent average. There does seem to be sort of bathub curve factor, though: if they make it past a certain point they seem to last longer. Lower power ones also seem to last longer. Sometimes they just go out, sometimes they begin rapidly flickering.
Maybe then it is heat related as you suggested; I'd lend more credence to that too because the bulb-population I have not had a single one go bad in despite years of use now is SANSI, which have weird-looking ceramic heatsinks. It does also seem to be worse in some fixtures than others (worst: bathroom vanities.) But even that is pretty damning: LEDs should have been designed for the fixtures people actually have. (I find the habit of creating LED fixtures also pernicious - there's a lot more waste when they go bad.)
Maybe then it is heat related as you suggested; I'd lend more credence to that too because the bulb-population I have not had a single one go bad in despite years of use now is SANSI, which have weird-looking ceramic heatsinks. It does also seem to be worse in some fixtures than others (worst: bathroom vanities.) But even that is pretty damning: LEDs should have been designed for the fixtures people actually have. (I find the habit of creating LED fixtures also pernicious - there's a lot more waste when they go bad.)