Any relevant deck can do 150 damage in its sleep. It's not that big a deal.
There is no argument for printing an OHKO attack without some gimmick, unless someone was purposefully trying to break every rule of good design.
It's...
kinda a factor in prize mapping. The decks you are using Frenzied Gouging against, you are expecting Gouging to be an even trade in prizes after all is said and done. But then you have to question what the opponent can put into the active to take a KO on Roaring Moon. This can be a difference between putting up a single prize pokemon or a double prize pokemon.
If Charizard can take those 2 prizes with a Charmander instead of promoting a Charizard, that's a safer play if the roaring moon user is not at 1 prize remaining. That's just one example, but that can apply to Charizard, Lost Zone Tina, Palkia, Bax/Pao decks that tech Palkia, and if you fail to knock off a Bravery Charm Miraidon can take advantage of that safe line of play too.
Rapid Strike Box also can easily abuse Roaring Moon at 30 HP by getting a moderate chance at Yoga Loop between Medicham, Radiant Alakzam, and Intelton VMax. That's probably a scary matchup no matter what.
Most other decks you are just taking KOs with Calamity Storm anyhow, and lesser powerful tank decks might be too linear to have a 1-prize pokemon that can clean up a 30-HP Roaring Moon.
That said,
you can still use Roaring Moon as a KO option and not need Emergency Jelly to make up for this, if you design your deck with that prize mapping in mind. This can either be saving Roaring Moon specifically for big KOs and playing a prize-efficient pokemon alongside it, or teching Roaring Moon as a one-of in something and not make it the main focus of the deck, saving only for specific pokemon to knock out at certain points in a game.
You really only need Emergency Jelly if you design your deck's major flaw around needing Emergency Jelly, which would be like playing a devout I-am-only-attacking-with-Roaring-Moon deck.
The factor of being at 30 HP left is more of a big deal than what PMJ is claiming, but the solution definitely does not have to be Emergency Jelly, nor is Emergency Jelly something Roaring Moon players should put on a golden pedestal.