Flatwounds… Now there;s something I think we can ALL agree don’t belong on a Strat! (Not because it’s not designed for it, but because so much of the appeal of a strat is their bright and clear attack. Though, of course, as I’m writing this, my curiosity is coming up a little…)
Fender swears the bullet ends promote stability by sitting more securely in the trem block than a round ball end. They also swear that their strings are integral to the “Strat” sound and it doesn’t sound like a Fender if you’re not also using Fender strings, but that, at least, is BS (and I used to fall for it, for the first couple years I played a Strat, so I’m not speaking from a lack of experience here). I’d say it seems like they could offer a minescule improvement in stability since the string can’t shift, but honestly a ball end isn’t going to really shift a perceptible amount anyway from where it settles once you tune it to pitch even if you’re routinely diving flat, and if you are your biggest source of instability is going to be the nut, followed by tuning pegs. If I was that worried, I’d spend $50 on a decent set of locking tuners rather than worrying about my ball ends.
(Actually, I think at this point all of my non-locking-bridge guitars DO have locking tuners, and a few locking nut/bridge ones, to boot, but that’s purely driven by how much faster they make string changes)