Realistically speaking, mixed attackers are just used wrongly.
People think that they're meant to use both stats at same time. No, the deal is that they *can* use both stats.
Which means that you can't prepare for both. If you followed OU recently, one of the reasons people at some point were calling for Iron Valiant's ban/suspect in the past as well as, say, Kyurem's, was specifically that you couldn't tell if they're phisical or special from team preview alone. You could send in Blissey only to be smacked by a Close Combat, but equally send in Slowbro and eat Shadow Ball instead. Similar discourse applies to Kyurem, send in a special wall for it to DDance in your face, or send in a phisical wall only to eat a Earth Power and get 1hkod or badly chunked. (Problem further exacerbated by Terastal obviously).
The "potential" to use both is much more impactful than "actually using both". Expecially as there's only so many moveslots a pokemon can use, so the freedom to use both sides of the spectrum is not "really" there.
(This obviously doesn't actually apply to current VGC tournament system, open team sheet removes the surprise factor, but you can still make use of this on the regular VGC ladder)
The real problem "mixed attackers" face isn't EVs, it's the fact that the mixed attack stats actually eat at their BST. Expecially with today's powercreep, if you want to be even remotely viable as attacker, you're looking at 120+ in atk or spatk (bar having some particularly busted signature move or ability to compensate a low base stat). But if your budget is 550 bst, and you're already allocating 250 of those in offenses, that means you are going to either be insanely slow or insanely squishy (or both for lower bst ends).
Actively using both stats hasn't really been a thing for dedicated sweepers in ages, other than to snipe a surprise KO, but surprise coverage isn't something strictly related to mixed attackers. Garchomp has run Flamethrower, Lando has run Grass Knot, heck people ran Hidden Power on mons with 50 special attack in some cases to snipe 4x weak enemies. But even on mon that had high secondary offensive stat, running mixed sets just was almost never optimal over just investing stab / stab / setup / coverage instead.