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Volatile cores deal way more damage than is reasonable and can neither be blocked nor suppressed. By my calculations based on what is required to survive one volatile core going off in your face they do somewhere around 60k fire damage on a t16.5 map with no damage boosting mods, My FRoSS occultist can survive one with dying sun up and then recoup enough to not get onetapped by surviving stragglers afterwards, I'm not certain any build other than the most ludicrous es stackers can survive two in close succession. If they were reasonable they would hit at most as hard as a shaper barrage ball(6 to 8k ele damage with some pen), rather than doing half to two thirds of the damage of the uber maven memory game (110 to 115k elemental damage split between the three evenly).
Drowning orbs are also shit because you can't see them and they give instant spells a cast time (no you cannot just frostblink out) even when you have immunity to minus action speed effects, if they didn't spam spawn and chase you they would be miserable, since they do they're intended solely to force you to sink more chaos into rolling maps and occasionally turning a risk map into a skip.
Having 90% all res, as well as sources of less fire dmg taken (arctic armour), less aoe dmg taken or fire to chaos on a CI build will reduce this number drastically, but your point still stands that only a small selection of build archetypes can reliable tank them more than once.
