P1N Warrior solo kill

[Axious]

Explain how it’s better when EW nascent is maxed at 10% heal. While ShB nascent capped at 25%?

EW nascent if you’re overpower combining. Roughly 3-5%. Decimate spamming roughly 6-8%. Inner chaos and Primal rend roughly 9-10%.

[Shurrikhan]

Explain how it’s better when EW nascent is maxed at 10% heal. While ShB nascent capped at 25%?

Neither has a "cap". It's just heal potency. The largest single GCD of single-target healing you could do in ShB was 670 via Inner Chaos (using IC's post-potency-squish numbers). Inner Chaos now does 600.
Fell Cleave, meanwhile, would do ~253 outside of IR or ~473 within IR. It now always does 400.
For sake of context, your lowest hit, Heavy Slash would previously have done 100p of healing; it now deals 400p, same as anything else other than Inner Chaos / Chaotic Cyclone.

The ST floor was raised tremendously, but the ceiling fell by only ~19% at most (2759 potency vs. 2400, or 2984 vs. 2400 if you include Onslaught and Upheaval). In AoE, both the floor and ceiling increased, since you have no AoE skill that deals over 800 relative potency per target. That "per target hit" clause, causing AoE healing to scale akin to ST throughput times target count instead of akin to AoE's damage itself, is what makes modern Nascent/Bloodwhetting so broken. In ST, an issue would come down to simple tuning; in AoE, it's procedurally busted.