252+ Atk Life Orb Excadrill Earthquake vs. 252 HP / 252+ Def Jellicent: 216-255 (53.4 - 63.1%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Excadrill Earthquake vs. 252 HP / 252+ Def Jellicent: 166-196 (41 - 48.5%) -- 71.1% chance to 2HKO after Stealth Rock, sandstorm damage, and Leftovers recovery
252+ Atk Life Orb Excadrill Earthquake vs. 252 HP / 252+ Def Mega Sableye: 148-175 (48.6 - 57.5%) -- 95.3% chance to 2HKO (people don't even run max defense either)
"spinblocking" amirite. a 1/3 of exca run Life Orb, and leftovers is negated in sand as well, so if rocks are up it loses to exca even if its not LO. Especially because almost half of exca run adamant 252 attack (1825 ou usage stats)
The only mon that truly spinblocks excadrill is gourgiest-super
252+ Atk Life Orb Excadrill Iron Head vs. 252 HP / 160+ Def Gourgeist-Super: 130-153 (34.7 - 40.9%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
As for starmie, you've already mentioned that it can't wall LO thunderbolt.
252 SpA Life Orb Starmie Hydro Pump vs. 248 HP / 144+ SpD Mega Sableye: 140-165 (46.2 - 54.4%) -- 54.3% chance to 2HKO (the spread people actually use)
Factoring in analytic, mega sableye does not exactly spinblock starmie either.
The same holds true for gourgeist and ice beam.
Also, Jellicent struggles to do much to sdef tran, which can shut it down with taunt, toxic it, and take maybe 30%~ from scald. Factoring in a potential protect, jelli gets worn down very quickly. It must also guess correctly between burning ferro to stomach power whips and taunt'ing to prevent hazards/leech seed to get the best matchup, because if it guesses wrong it's taking a solid 70%~ from power whip. Wisp can also miss.
Finally, mega zam pretty freely just CMs on jelli and 2hkos with shadow ball; the matchup vs mattybrolic's team is being vastly simplified.
Besides, this "huge" hazard stacking matchup is also being blown out of proportion. Ferro has the potential to put a big dent in you, and its subpar "spinblocking" abilities aside, you have to remember that pretty much every water type has this "huge advantage" as most rock setters (ferro aside, which jelli isn't gauranteed a great matchup against) are going to be falling to the big, bad waters of OU: keldeo, manaphy, and azu.
You mention it "breaks defensive cores" lol. It has 60 base speed which falls short of heatran, gliscor, mew, mvenu, rachi, and skarm, and it can't touch mega-sableye outside of a scald burn, and still loses to grasses like celebi and amoong. One thing you forgot to mention is that it's a "stallbreaker" that can't be crippled by goth, unlike most other taunt+status users like mew, tran, and mandi, but it doesn't make much of a difference when heatran can just outpace you, toxic you, and wall you with the rest of the team. There's more to stallbreaking than taunt+status/setup.
Despite all the trash i talked about jellicent, it's a mon that keld can't just scald its way through, and has reliable recovery+some bulk. At any rate it's much better than D rank crap so i'm not against a raise, I just think that jellicent is really being blown into something its not.