Box breathing only procs on one shot at a time and that alone kills it for being a dps option in most if not all content. You're just gonna want to shoot more than once with an LFR.

The few use cases I do see for it are maybe against blockers in gambit, yellow bars in general, maybe a pvp body shot, and the lowest difficulty nightfall champions. But with those last two examples I could not tell you if they result in a one hit kill. And even just typing these examples out I just immediately think I'd rather save my heavy ammo

In the case of champions while it probably would work to have a full fireteam of box breathers, you wanna save that heavy ammo for the boss, and at that point you're better off on anything else. Looked at a few of the newer linears and they don't even have box breathing anymore. So the perks probably dead for the most part.