Neither are glass cannons but the fact remains Volcarona has more defensive utility and runs bulky sets often. Plus, it resists U-Turn and can cripple physical attackers with Flame Body.
The issue is that things with Moon's power and speed tier are typically supposed to be balanced by low defenses. This isn't exactly the case, so I don't know why so many folks seem to think it is balanced.
Volcorona's base 100 speed tier is not fast enough in gen 9 for a +1 to be nearly as immediately threatening. It has some defensive utility, but it isn't exactly bulky without investment.
Volcarona is not susceptible to our priority options at all:
Grassy Glide - 4X resist. Threatens Rilla back with death and burns
Extremespeed - threatens burn to Dragonite and doesn’t do much to bulky Volcs until Dragonite obtains boosts. Furthermore, Dragonite doesn’t want to Tera here as it loses its typing advantage.
Ice Shard - Resists and threatens burn. Bulky Volc is a soft Weavile check
Sucker Punch - Volc threatens burn or further set up and kills Kingambit unless it Teras
Thunderclap - set up fodder. Volc is at +2/+2/+2 and probably ran away with the game already
Bullet Punch - lol. Volc takes a dump on Scizor.
Aqua Jet - doesn’t really exist and users Sam-H / Azumarill / Crawdaunt / Barraskewda all lose to coverage and Tera options.
E-speed Dragonite is actually pretty good against Volc and can often use Volc as setup fodder itself. Sucker Punch also does a lot of damage except in the case of like Tera Fairy. You do play some Sucker Punch mind games, but Volc isn't exactly taking dark STAB from Gambit very easily.
Both Dragonite and Kingambit hate burn, but this is a roll, not a guarantee. Again, crippling one sweeper or revenge killer to avoid being swept seems reasonable. And the crippling is just a chance, far from a guarantee.
Aqua Jet primarily fell off due to the combination of Waterpon and all the Dragons in the tier. In prior iterations of the metagame, it was enough of a threat for Volc to often run Tera Grass. I could see Aqua Jet coming back if Waterpon gets banned.
Meanwhile, Roaring Moon's natural typing resists Grassy Glide, Aqua Jet, Sucker Punch, Thunderclap, and Shadow Sneak. Only Ice, Fighting, Normal priority are really reliable to hit Moon neutrally or super effectively. I guess Bullet Punch, but Scizor isn't exactly the best these days. Tera Flying counters Fighting and the very rare First Impression while providing strong offense. Other Tera options could cover other things. Steel used to be common for E-speed, Ice Shard, and Bullet Punch, for example, and could be used to hit fairies.
I do think both of these pokemon are problematic in a Tera metagame because they can use it to get an extra turn to setup and snowball. Priority that you might bring to check one or the other won't necesarrily be reliable. But if you are asking me which one is more resistant to being revenge killed after only 1 turn of setup, it's clearly Roaring Moon. It has better stats for bulk, a far superior speed tier, and a greater number of important natural priority resistances, unless you want to count like First Impression as important. You can also often use a Choice Scarf or Energy Boost speed mon to outspeed a +1 Volc. This doesn't happen much at Moon's speed tier. It's often priority for Moon or nothing, and it resists too much priority.