Rimfire rounds are manufactured by having the primer element dropped into the case while spinning it to force the primer into the rim. While this tends to create a pretty even dispersal into the rim, it isn't an exact science. So, there will always be the possibility that the portion of the rim smacked by the firing pin might have a weaker charge, and it may even be insufficient to light off. And, it does take quite a hard smack to set off even a "strong" charge, as it has to flatten that section of rim.
That being said, I have fired tens of thousands of rimfire ammo over the last 27-ish years. Most of it has been CCI MiniMags, simply because, during the first several years, that was all that was typically stocked at the local department stores, and I didn't know that cheaper stuff even existed. Later, that trend continued because my small autoloaders, which comprise most of my rimfire battery, won't cycle the "bulk" stuff reliably.
I have never had a misfire with rimfire ammo.
I did have one with about as many centerfire rounds, and it was a commercially-reloaded round of .38 Special ammo that also happened to be about fifteen years old. It fired the second time the hammer (of a Charter Arms Undercover, made in 1987) was dropped on it.
