All ACU die to any T4 that isnt an ML, regardless of upgrades. Upgrades like double nano + splash are more than capable of being countered by things such as mega. Singular nano and splash is often rushed on 5 t2 pgens to abuse at around the cusp of the t3 stage in teamgames and can be difficult to stop, but that's because the strat is difficult to stop for any faction really. You just build PD, range bots, and abuse air to minimize vet.
Walking around on front with ACU shield when enemy has 2 or more experimentals is just retarded regardless of faction. So any of these t4 assault situations are certainly defensive. Except maybe Seraphim.
Cybran have like 6 trillion different ways to respond to any of the situations you describe and aren't forced to hail mary an ACU, including the fact they have the objectively best land t4 for some reason.